XDA Forums are your main source of information. Youtube also helps a lot with rooting & flashing ROMs because you can actually see what's happening on your phone and computer (when rooting). Every phone is different when it comes to rooting and flashing ROMs but I'm sure you'll have no trouble finding youtube videos for a Galaxy S. You can try that first or learn the common terminology when it comes to Android hacking:
bootloader - a "program" if you will, that runs before the phone's operating system...tell you the truth, I still don't have a clear explanation of what a bootloader is, I do know it has to be unlocked before rooting & flashing.
recovery - or stock recovery, is a program that can be accessed on your Captivate by turning off the phone, then holding volume-up, volume-down, & power button at the same time until your phone boots into recovery mode. Recovery allows you to install updates manually, & delete system cache (return to stock)
ClockWorkMod recovery - similar to stock recovery but has more functions. It's installed during rooting or after using ROM manager. This recovery is generally preferred over the stock recovery because of more functions.
flashing - pretty much the same as installing
Superuser - a powerful app that is installed automatically after rooting. Allows you to access system-level files and apps and modify them. (sorry I can't be more detailed with it as I haven't really played with Superuser that much)
ROM - in layman's terms, an operating system. It's still going to be Android but different ROMs give you different options on customization and/or performance. See a list of changes a particular ROM makes in that ROM's thread on XDA.
ROM manager - an app that can be installed after rooting. Allows you to backup your ROMs, search for other ROM's, update them, and install/flash new ROMs all in a convenient and easy-to-use interface.
Titanium Backup - another powerful app that can be installed (and should be) after rooting. It allows you to backup your apps along with all their settings and progress in games. Get this before installing your first ROM and back up the apps that you use frequently. Trust me, it'll save you the time & trouble of changing the settings back to what they were again and again after flashing new ROMs.
Hope that helps.