What is a Custom ROM and what it's usefulness?
A Custom ROM is the equivalent of a CFW for jailbroken PS3 consoles. It's a modified Android OS/firmware. It allows greater control than the original (stock) OS/firmware your phone has. Most of the time is heavily modified to suit the needs of the developer or user.
How many ROMs exist?
Infinite but they are categorized
1) AOSP= Stands for Android Open Source Project its the most common type of ROM every phone has at least one custom ROM of this category. Also most stock ROMs are AOSP types like Google's phones.
2)Touchwiz= It's Samsung's own customized ROM, its being developed by Samsung only and for Samsung phones only, It can be edited and ported to different Samsung devices with ease.
What will I need?/ Installation steps
Let's get to the nice stuff. First of all your phone MUST have a custom recovery. Either follow my previous guide You must login or register to view this content., or google "How to install custom recovery on (your phone's model)" It's an easy process.
After that part is done all you need is the file of your favorite ROM (Lineage, Ressurection Remix, etc) saved in your internal or external Storage of your phone.
Boot into Recovery Mode (Varies across manufacturers but typically is Power button + Volume Up or for most Samsung devices Power button + Home button + Volume Up button)
(SUGGESTED)Make a backup of your phone
(SUGGESTED)Wipe Data, Dalvik and Normal Cache
Hit the Install button
Find your file and select it
Confirm the installation
Once it finishes wipe cache and reboot
Wait till it's fully booted
Configure your new ROM
Relatively small guide but there are not much to be said here. That's basically everything you need. Before installing a ROM read it's features etc. There are literally hundreds of ROMs for each phone so best to take your time and see what it's has to offer and how the community of users love or hate. As always if this thread helped you, my part has been fulfilled. Enjoy another guide (Still a noob at making them) by EvolveRebxg. Have Fun people!