With Android you don't jailbreak, you root. Rooting allows your phone to control things its not supposed to, sort of like jailbreaking. You must root first before you install roms.
ROMs.
Roms are basically like different distros of Linux (Ubuntu, Debian, Fedora, openSUSE, etc.) and kernels work the exact same way.
Samsung phones use a program called ODIN (Heimdall is another, but ODIN is made by Samsung) to hard flash different roms, recoveries and kernels. If you have a custom recovery already you can flash stuff from recovery. Otherwise you have to put your phone in "download mode" (basically the same thing as putting an iPhone/iPod into DFU mode) and connect it to ODIN/Heimdall.
Roms can do things like system wide themes, remove bloatware (preinstalled carrier software), overclock, improve performance in other ways besides overclocking, add things from different phones, etc.
The Galaxy S (I have one) has a stuff ton of support because it's a part of the Galaxy line. The latest version of Android-- 4.0 Ice Cream Sandwich-- is available for the Galaxy S as a custom rom, I have it and it runs great.
CyanogenMod is an AOSP rom (AOSP meaning it was built from the ground up and is not based off a Samsung rom) that allows a loving stuff ton of customization options.
MIUI is another AOSP rom by a Chinese development team that completely redesigns the layout and the launcher (see below) is more like an iPhone.
Launchers
Launchers are your homescreen, it's what you see when you press the home button. Android lets you change your launcher to whatever launcher you want (provided the developer supports your phone). My favorite is GoLauncher EX which is free. There's also ADW, LauncherPro, SPB Shell ($15 though), PandaHome, Lightning Launcher, and countless others. Most launchers are free-- or at least have a free version-- but some are pay only (Like the outrageously priced SPB Shell, ugh).
The Market
Android has the Android Market, which is Android's Appstore. Nothing much to say about it other than it's an app store.
Dangers
You MUST know what you're doing before you flash ANYTHING. Flashing can brick your phone and you'll lose everything. Fortunately the Galaxy S is nearly unbrickable and there's countless ways to get it back unless you forgeted up the hardware itself. I have a jig as a backup measure, you stick it into the bottom of the phone and it puts it into download mode even if everything on the phone is absolutely forgeted up beyond belief. Some phones, like the Nexus S, have locked bootloaders that prevent you from flashing anything. Unlocking can corrupt your IMEI number and there's absolutely nothing you can do about it, you're left with a fancy paperweight. Fortunately again, the Galaxy S has an unlocked bootloader straight out of the box.
If you have any questions about Android feel free to ask me, I've been with Android for a while and I'm starting to get into Android development.
