Care to explain a little more in depth?
Also I have an SIII
Custom ROMs based on the Android open source project code (AOSP if you see that and wonder). Some are just small tweaks to a stock android, some are heavily modified. One you have root installed and clockwork mod recovery installed, installing is simple. Just format /system, /data, and /cache, wipe the dalvik cache, and install.
They can get you upgrades past what the phone manufacturer supports. For instance, I THINK Chief's phone stopped at 2.3 (Gingerbread) but he was able to get 4.0 (Ice Cream Sandwich) and 4.1 (Jellybean) by installing CyanogenMod 9 and 10, which are based on android 4.0 and 4.1, respectively.
My phone hasn't upgraded to 4.1 yet either but I have it through CyanogenMod 10 as well.
