1. HBOOT - Different versions, How it works, how to update it, etc.
2. Recovery - Different versions, How it works, how to update it, etc.
3. System Partitions - System, Cache, SD-EXT, etc. - What each partition contains
4. Kernels - Different versions, how to load/replace
5. Radios
Not sure how technical you want this info, so I'll keep it simple.
1) HBOOT is the bootloader. On the Evo, you can get to it by powering off your phone, then hold power and vol-down together until the white bootloader screen appears. There are two versions: the Release bootloader and the Engineering bootloader. Release is what we all got out of the box. Depending on your root method, you may have the Engineering bootloader. To fully unlock/root/jailbreak your phone, you want the Engineering bootloader. Not essential anymore ever since unrevoked forever and unrevoked 3.2x came out, but some custom ROMs require the extended fastboot commands available only with the Engineering bootloader.
2 and 3) The phone's internal memory (not the SD card) is solid-state (flash) memory, AKA NAND. It can be partitioned much like a normal PC hard drive can be partitioned. The bootloader exists in its own partition. Recovery is another partition, radio, system, cache, etc are all partitions. The recovery image is the program that is written to the recovery partition. It's purpose is to recover your phone's system in case something gets corrupted. For the Evo, there are 2 choices for custom recovery images: clockworkmod and RA. Both have their uses. They give you the ability to flash ROMs. They also allow you to clear cache and make a NANDroid backup/restore (an image of all the partitions on the NAND flash memory). Here are the standard partitions on an Android phone:
/misc - not sure what this is for.
/recovery - the recovery partition, explained above
/boot - bootloader, kernel
/system - operating system goes here: Android, Sense, boot animation, Sprint crapware, etc
/cache - cached data from OS usage
/userdata - user applications, data, settings, etc.
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.
When you do a factory reset (AKA: wipe, hard reset, factory wipe, etc.), you are erasing the /userdata and /cache partitions. Note that a factory reset does NOT put your phone back to its factory state. 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 it really is a poor choice of words.
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.
4) Operating systems and kernels go hand-in-hand. Some custom OSes will require a particular custom kernel that was built for that OS. Some kernels are intended for universal use (compatible with any OS). 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, effectively overwriting it. A popular combination is a rooted stock ROM with a custom kernel. For the Evo, the hot features of kernels are:
-- a fix to remove the 30fps cap imposed by HTC and improve touch tracking sensitivity (though the latest OTA update includes an HTC kernel that also fixes these problems)
-- 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 tho)
-- HAVS (a control system that dynamically adjusts the voltage based on CPU load). This has proven to be huge for battery life.
5) Radios are in their own separate partition. I don't claim to know much about this. NANDroid backups do not back up the radio partition. AFAIK, there are no custom radio ROMs to flash. As long as you have the latest stock radios, you are good to go.
