What is a Galaxy S4 custom ROM?
Custom ROMs are modified versions of stock firmware or even completely built-from-scratch aftermarket firmware for your Galaxy S4. You will need a rooted Galaxy S4 and also a custom recovery (such as ClockworkMod or TWRP recovery) installed to install custom ROMs.
Why install custom ROMs on the Galaxy S4?
Custom ROMs can add new features such as multi-window enabled for all apps, add overclocking to your Galaxy S4, and even save battery life.
Basically, think of it as a modified car to make your Galaxy S4 function better. Also, with custom ROMs you will be able to install the latest Android operating system without waiting for Samsung to release an official version. (E.g. When Android 5.0 is out, you will be able to install it months before Samsung releases their own version through AOSP ROMs.)
Any questions? Leave in the comments box!