NOTE :- This Method will work in most cases, but make sure you take responsibility of whatever happens with your phone :) [Although nothing will happen :-P]
Now many of us use Android phones, the system we use is called as STOCK ROM, since it is given to us by the manufacturer.
Sometimes to enable additional functions or have different looks, themes or performance we might think of a different OS [ROM]. Custom ROM's are made with this in mind, usually they save a lot of your Internal space by removing all the unwanted apps, secondly there are performance improvements, many different functions etc.
Question arises how to get the custom ROM and How to Identify If it is good ?