superuser is an app that allows you to access system files to edit them.
if you are rooted, your biggest "cool" thing to do is use a custom rom. i personally use fission. follow this link to learn about the rom >
http://www.droidforums.net/forum/fisson/109906-rom-fission-2-4-3-droid2-vanilla-generic-android-aosp-like-fission-rom-manager.html
but here is a basic overview of what you can do
ROM: a rom is basically a new operating system so to speak. updating to a custom rom is like going from XP to vista (as an example.) the same basic features are there, but have been modified and altered for ease of use or other reasons
Theme: a theme is installed "on top" of a rom. it installs a pre-set bundle of images to replace ones already in the system (i.e. changing title bar color, text color, keyboard, etc)
Recovery: revocery mode is what is used to install any system files. because files are always in use by the OS, you need to boot into recovery so you can replace ALL of the files that need to be replaced. this is kind of like command prompt on a windows PC .
Clockwork: Clockwork is (in my opinion) the most common and easiest recovery to use. it is flashed using the "rom manager" app in the android market. once the recovery is installed, you can basically do anything yo want to your phone. be careful tho, deleting the wrong files can brick your phone.
SBF: SBF is the only way to get out of a bricked phone (assuming you can still get to bootloader) it is wiping and installing all files on your phone. this is the same concept how the manuf. programs it before shipping.
Bootstrapper: the bootstrapper is only needed for droid 2, droid x, and a few other phones. the reason a bootstrapper is needed is because motorola locked down the stock recovery and bootloader, so a custom revocery cannot be "installed" what the bootstrapper does is injects code into the startup sequence that tricks the phone into booting into custom recovery (clockwork)
hope this helps..... let me know if you are confused about anything