I don't have much knowledge of clockwork mod or super user, but I'm sure the other members here can tell you what they do.
EDIT: I forgot to mention also, with root you can remove carrier apps taking up space too.
Just be careful what apps you remove.
Clockwork mod in a nutshell (sort of)
Well, this is actually Amon Recovery for the Droid Eris. But the basic functionality and terms are essentially the same. The menu is pretty primitive (there is no touchscreen support, so you control the menu's either by the volume keys or some other buttons on the phone).
Highlighted is the "reboot system" function. Does exactly what's on the box.
Backup/restore - THIS IS VERY IMPORTANT. It basically makes an identical copy of your current Android system (apps, user data and all) and saves it to the SD card. Its very important to do this before you do any major changes to the phone. That way, if something goes bad, no big deal, just restore the backup and your right back where you left off. They tend to be pretty big and take awhile to do though. But its your safety net when you tinker.
Flash zip from SD card - Simple. You put a ZIP file that you download onto your SD card (which usually contains a ROM), select the zip, press confirm, and let the device do its thing. This is how you install custom ROMs, scripts, and other neat things.
Wipe - Used for wiping the data partition (it contains app data and such) and Dalvik cache (used for optimising apps to run faster). Its sort of like performing a factory reset, except your SD card remains intact. You normally do this after flashing a new ROM, since stuff from the old ROM can conflict with the new one.
Partition SD card - This is an antiquated feature and a piece of Android history so its no longer really needed or used. Its from back before Android 2.2, before you could store apps on the SD card. And back then, you only had 100 MB TOTAL for apps. Some people figured out if you divide the SD card up right, you can store apps on the SD card and get a lot more space, and this was called "Apps2SD".
Other - More debugging like stuff
Power off - Do I really need to explain this?
Depending on what device you use, some features are added and removed.
And super user is pretty simple. Its essentially like gaining administrator priviledges in Windows. Your word becomes god's to the machine and will do whatever you tell it to.