Generally speaking custom roms fall into 2 camps: modded stock roms, which are modified versions of the phones original software; and original roms which tend to be built from scratch for a number of models and tend to be based around AOSP(Android Open Source Project), basically barebone android without any manufacturer additions. Both have benefits and pitfalls. Modded stock roms are not hugely different from what you already have; they can be faster, more customisable, and generally less bloated than stock but
at the same time they are changing something that the OEM has worked on for probably over a year, and what may seem to be an improvement in a devs eyes may not be to your taste. The speed can come at the cost of smoothness, stability, or battery life and possibly all three. Original roms such as CyanogenMod, AOKP, Omni, MIUI etc are designes normally from the AOSP source, not the manufacturers rom, so they will have the latest available version for your phone and have no carrier/manufacturer bloatware but
also no original firmware, which means generally that apps designed specifically for your phone (camera, music, and so on) will not work. Plus once again, what some people define as better may not be your cup of tea.
I'm not familiar with Samsung devices, and generally all phones have slightly different versions of the process, but before you do anything you should root your phone, flash a custom recovery and make a nandroid backup so that you have your original set up available to restore; also back up your app data using Titanium backup or something similar. The reason for this is, when you flash a rom it will overwrite your current firmware and you'll probably be required to wipe user data too.
This is just a very rough guide, the best idea is to read the rom development thread to find out if there are any potential problems. Good luck anyway.
— modified on Apr 5, 2014, 7:37:08 PM