Rom Manager can be used for backing up the complete contents of your internal phone SD to a ROM (think of it like Norton Ghost, for your phone). In order to be able to reinstate a backup it can also install ROMs- both your own backups, and ROMs provided by other sources. Some recovery utilities are included. On the Captivate, you'll need to install a recovery kernel (Clockwork). Requires root (doesn't mention it on the app's page in the android market, but it's in my list of apps allowed SU access). Installation of the Clockwork kernel is the very top button in the Rom Manager screen, titled "Flash ClockwordMod Recovery".
Titanium Backup backs-up, installs and manages applications (it even allows you to halt, and restart system apps). It's useful if you want to quickly transfer favourite / purchased apps to a new ROM. Root is required.
Note that both utilities, by default, store their backups on the phone's internal storage. This is the worst place for your backups to exist. I recommend that you change the default location / copy those backups somewhere else (to a PC, for example).
Both apps are must-have's in my opinion, but they do different things, and it's important to understand the differences.
Hope this helps, consider clicking Thanks, it makes me smile.
AoN
P.S. Just saw your extra post, and for rooting, I think the easiest way is to use this:
http://forum.xda-developers.com/showthread.php?t=803682
...the "get unlock code" will let you know what the code is to unlock your phone, so that you can use any SIM card in it. The code for each phone is hidden in a file, on the phone, in the Eclair build. Note that as soon as you upgrade to any Froyo-based ROM, that information is deleted.