Reboot
A reboot, also called a soft reset, is when you completely turn your phone off and back on again. This is often the first step you will be given in troubleshooting problems with your phone as most issues are related to memory or a temporary glitch that gets resolved through a refresh of the system. I tell people that they should do a reboot once a week as it assists in reducing lag.
Battery Pull
A battery pull is something that you should not do often when the phone is powered on, but may be necessary. When your phone becomes frozen and will not recover regardless of buttons pressed or giving it time you will have to access your battery and pull it (thus the name). This should be done as a last resort.
Hard Reset
A hard reset is not for the faint of heart as it resets the phone to the factory settings. This means you will have to redo all of your settings and customization while waiting for the Android Market to reload your apps. This can be very time consuming (you can reduce the time by having a backup app).
Sideloading
Is just like it sounds, its loading apps from the side, aka loading them through an SD card, or any where that isnt the Market.