This is a rewrite/repost, condensed and updated with information that I think is useful to a lot of rooted users. Partitions and kernels aren't frequently discussed. Most of us know that they exist but don't really know how they work. Hopefully it all becomes clear after reading this 
The phone's internal memory (not the SD card) is solid-state (flash) memory, AKA NAND. It can be partitioned much like a normal hard drive can be partitioned. The bootloader exists in its own partition. Recovery is another partition; radio, system, cache, etc are all partitions.
Here are the standard partitions on an Android phone:
/misc - not sure what this is for.
/boot - bootloader, kernel
/recovery - holds the recovery program (either clockworkmod or RA recovery for a rooted Evo)
/system - operating system goes here: Android, Sense, boot animation, Sprint crapware, busybox, etc
/cache - cached data from OS usage
/data - user applications, data, settings, etc.
The below partitions are not android-specific. They are tied to the hardware of the phone, but the kernel may have code allowing Android to interact with said hardware.
/radio - the phone's radio firmware, controls cellular, data, GPS, bluetooth.
/wimax - firmware for Sprint's flavor of 4G, WiMax.
During the rooting process, a critical piece of the process is disabling a security system built into the bootloader that protects these partitions from accidental (or intentional) modification. This is what's referred to as "unlocking NAND." The security system can be set to active or inactive. S-ON means the security is in place (NAND locked). S-OFF means the security is off (NAND unlocked). When S-OFF, you have the ability to modify all partitions. With S-ON, you only have write access to /cache and /data. Everything else is read-only.
When you flash a custom ROM, that ROM typically includes a kernel and an OS. That means the /boot and /system partitions will be modified at a minimum. Some ROMs require a clean install, so a format of the /data and /cache partitions is sometimes built into the .zip that you flash. This is essentially doing a factory reset. See next paragraph.
When you do a factory reset (AKA: wipe, hard reset, factory wipe, etc.), you are erasing the /data and /cache partitions. Note that a factory reset does NOT put your phone back to its factory state from an OS standpoint. If you've upgraded to froyo, you will stay on froyo, because the OS lives in /system, and that is not touched during a factory reset. So "factory data reset," as it says under Settings > SD & phone storage, causes confusion. It's not a factory reset. It's a factory DATA reset. Now you know the distinction.
The SD card can also be partitioned to include a section dedicated to storing user apps. To create the partition, your SD card needs to be formatted. Typically a user will copy all the contents in the SD card to a PC hard drive, wipe the card and partition it, and then copy everything back.
Onto kernels....
A kernel is a layer of code that allows the OS and applications to interface with your phone's hardware. The degree in which you can access your phone's hardware features depends on the quality of code in the kernel. The homebrew (rooting) community for HTC has made several kernel code improvements that give us additional features from our hardware that the stock kernel does not. When you flash a custom ROM, you automatically get a kernel. But you can also flash a standalone kernel ROM on top of the existing one, effectively overwriting it. These days, the difference in custom kernels is less about new features and more about alternate configurations. Choosing a custom kernel is basically choosing one that works best with your ROM.
Kernel developers are typically responsible for a small, specific feature in the kernel. For example, netarchy's contribution to custom kernels mainly revolve around removing the framerate restriction that was present in the stock HTC kernel. However, because of the open-source philosophy of all the devs, each kernel "distribution" contains the work of several devs. They openly share new code that they deem will benefit everyone. Basically the best features stay in, and the distributor of the kernel will give credit where credit's due. This means that for any custom kernel you try, you might be using netarchy's FPS fix. It's all a team effort. A new improvement will make it to most if not all of the various kernel distributions in a short amount of time.
For the Evo, the hot features of custom kernels are:
-- a fix to remove the 30fps cap imposed by HTC and improve touch tracking sensitivity (HTC finally fixed this in their stock kernel late last year)
-- disabling perflock to enable CPU throttling. Great for increasing the performance of your phone and/or improving battery life
-- iptables firewall to enable wifi-tethering via the wifi-tether app
-- 3 or 5 point multitouch support (not too many practical applications)
-- HAVS, a control system that dynamically adjusts the voltage based on CPU load. This has proven to be a battery saver, but it can actually have the opposite effect when multiple control systems are operating (like setCPU).
-- BFS kernel task scheduler as an alternative to the standard CFS
-- SBC (the ability to charge your battery beyond the default safe limit). The concept is similar to overclocking a processor: you're overriding the safety limits established to achieve additional performance. The benefit here is that you may gain more use of your battery per charge. The drawback is that you can damage the battery and significantly reduce its longevity. Some kernels claim they are using a safe technique to prevent battery damage. Just be aware of the potential risks.
Note about 4G and HDMI:
AOSP ROMs (ROMs based on pure Android without 3rd party manufacturer's code) are currently lacking robust 4G and HDMI support. This is because 4G and HDMI are hardware-specific features, not Android-specific, so there is no existing kernel code in the Android Open Source Project (AOSP) that is specific to 4G or HDMI support.
HTC is the manufacturer of the phone, so it has intimate knowledge of the hardware it uses in the phone; therefore the kernel that HTC has written does support HDMI and 4G. The problem is that the kernel is also tied in with their Sense UI that it's not possible to simply extract the portions of the code governing 4G and HDMI. Therefore, Sense-based ROMs support 4G because they simply borrow the existing HTC code. Meanwhile AOSP ROMs have to write their own code for 4G and HDMI in order to support it. Their ability to do this depends on how much info they can obtain about the hardware components in the phone and any drivers they may be able to find. Otherwise, it's really a task of reverse-engineering the hardware, which can be a thankless, time consuming task.
For a more in-depth read into how partitions work in Android/Linux, take a look here, courtesy of Akazabam:
http://androidforums.com/evo-4g-all...ore-information-about-android-partitions.html
The phone's internal memory (not the SD card) is solid-state (flash) memory, AKA NAND. It can be partitioned much like a normal hard drive can be partitioned. The bootloader exists in its own partition. Recovery is another partition; radio, system, cache, etc are all partitions.
Here are the standard partitions on an Android phone:
/misc - not sure what this is for.
/boot - bootloader, kernel
/recovery - holds the recovery program (either clockworkmod or RA recovery for a rooted Evo)
/system - operating system goes here: Android, Sense, boot animation, Sprint crapware, busybox, etc
/cache - cached data from OS usage
/data - user applications, data, settings, etc.
The below partitions are not android-specific. They are tied to the hardware of the phone, but the kernel may have code allowing Android to interact with said hardware.
/radio - the phone's radio firmware, controls cellular, data, GPS, bluetooth.
/wimax - firmware for Sprint's flavor of 4G, WiMax.
During the rooting process, a critical piece of the process is disabling a security system built into the bootloader that protects these partitions from accidental (or intentional) modification. This is what's referred to as "unlocking NAND." The security system can be set to active or inactive. S-ON means the security is in place (NAND locked). S-OFF means the security is off (NAND unlocked). When S-OFF, you have the ability to modify all partitions. With S-ON, you only have write access to /cache and /data. Everything else is read-only.
When you flash a custom ROM, that ROM typically includes a kernel and an OS. That means the /boot and /system partitions will be modified at a minimum. Some ROMs require a clean install, so a format of the /data and /cache partitions is sometimes built into the .zip that you flash. This is essentially doing a factory reset. See next paragraph.
When you do a factory reset (AKA: wipe, hard reset, factory wipe, etc.), you are erasing the /data and /cache partitions. Note that a factory reset does NOT put your phone back to its factory state from an OS standpoint. If you've upgraded to froyo, you will stay on froyo, because the OS lives in /system, and that is not touched during a factory reset. So "factory data reset," as it says under Settings > SD & phone storage, causes confusion. It's not a factory reset. It's a factory DATA reset. Now you know the distinction.
The SD card can also be partitioned to include a section dedicated to storing user apps. To create the partition, your SD card needs to be formatted. Typically a user will copy all the contents in the SD card to a PC hard drive, wipe the card and partition it, and then copy everything back.
Onto kernels....
A kernel is a layer of code that allows the OS and applications to interface with your phone's hardware. The degree in which you can access your phone's hardware features depends on the quality of code in the kernel. The homebrew (rooting) community for HTC has made several kernel code improvements that give us additional features from our hardware that the stock kernel does not. When you flash a custom ROM, you automatically get a kernel. But you can also flash a standalone kernel ROM on top of the existing one, effectively overwriting it. These days, the difference in custom kernels is less about new features and more about alternate configurations. Choosing a custom kernel is basically choosing one that works best with your ROM.
Kernel developers are typically responsible for a small, specific feature in the kernel. For example, netarchy's contribution to custom kernels mainly revolve around removing the framerate restriction that was present in the stock HTC kernel. However, because of the open-source philosophy of all the devs, each kernel "distribution" contains the work of several devs. They openly share new code that they deem will benefit everyone. Basically the best features stay in, and the distributor of the kernel will give credit where credit's due. This means that for any custom kernel you try, you might be using netarchy's FPS fix. It's all a team effort. A new improvement will make it to most if not all of the various kernel distributions in a short amount of time.
For the Evo, the hot features of custom kernels are:
-- a fix to remove the 30fps cap imposed by HTC and improve touch tracking sensitivity (HTC finally fixed this in their stock kernel late last year)
-- disabling perflock to enable CPU throttling. Great for increasing the performance of your phone and/or improving battery life
-- iptables firewall to enable wifi-tethering via the wifi-tether app
-- 3 or 5 point multitouch support (not too many practical applications)
-- HAVS, a control system that dynamically adjusts the voltage based on CPU load. This has proven to be a battery saver, but it can actually have the opposite effect when multiple control systems are operating (like setCPU).
-- BFS kernel task scheduler as an alternative to the standard CFS
-- SBC (the ability to charge your battery beyond the default safe limit). The concept is similar to overclocking a processor: you're overriding the safety limits established to achieve additional performance. The benefit here is that you may gain more use of your battery per charge. The drawback is that you can damage the battery and significantly reduce its longevity. Some kernels claim they are using a safe technique to prevent battery damage. Just be aware of the potential risks.
Note about 4G and HDMI:
AOSP ROMs (ROMs based on pure Android without 3rd party manufacturer's code) are currently lacking robust 4G and HDMI support. This is because 4G and HDMI are hardware-specific features, not Android-specific, so there is no existing kernel code in the Android Open Source Project (AOSP) that is specific to 4G or HDMI support.
HTC is the manufacturer of the phone, so it has intimate knowledge of the hardware it uses in the phone; therefore the kernel that HTC has written does support HDMI and 4G. The problem is that the kernel is also tied in with their Sense UI that it's not possible to simply extract the portions of the code governing 4G and HDMI. Therefore, Sense-based ROMs support 4G because they simply borrow the existing HTC code. Meanwhile AOSP ROMs have to write their own code for 4G and HDMI in order to support it. Their ability to do this depends on how much info they can obtain about the hardware components in the phone and any drivers they may be able to find. Otherwise, it's really a task of reverse-engineering the hardware, which can be a thankless, time consuming task.
For a more in-depth read into how partitions work in Android/Linux, take a look here, courtesy of Akazabam:
http://androidforums.com/evo-4g-all...ore-information-about-android-partitions.html
(sgs+)
