roms, how do they work?
In all seriousness, I've never used android. What are they? Modifications to the OS?
I'll give a rundown of what they are. Basically its a complete change to your operating system, in his case he came from touchwiz so there will be tw based roms, AOSP based which will be more common and CM based. They all bring different features, IMO AOSP is best for if you want fast, and PA is a good example, CM is more features at the cost of stability but faster updates.
There is two types of flashing, dirty flashing and clean flashing. Clean flashing is where you wipe System, Data, Dalvik and Cache. An example of dirty flashing would be for a system update and that's where CM comes into play, CM has an auto updater for if you flash it and it makes it easy, for any other roms you would flash over the current ROM, wipe dalvik/cache and restart, system will prompt that its upgrading and boom you're done
You can check several websites which I will not list due to the forum linking rule, but you can look for yourself. There will be plenty of stuff for his phone since its Samsung and they have the most marketshare (iirc?). I've helped people with installing roms on many phones but not Samsung so you'd have to look for yourself. The install is painless but stressful at least for me, and make SURE you read and read before you do anything if you want to flash a rom ever.
Theres two types of bricking, hard bricking and soft bricking. I've experienced both, hard bricking is not pretty. It makes your phone a paperweight but its next to impossible unless you REALLY forget up, soft bricking is common and is really easy to fix, until you find out you don't have a backup which you'd have to sideload and its convoluted and you'd have to look for yourself on the phone and blah blah blah I'm not going into any more detail. If anyone wants to look further then they can but installing roms isn't too convoluted, rom features are listed in the OP and I'll let you read that if you really care to.