The term "ROM" used by mobile maniacs is not the OS. ROM's are changing for different appearance as well as user interface. The OS will be remaining the same even after flashing different ROM. There are two types of ROM
- Stock ROM - The ROM comes along with the phone
- Custom ROM - Created by different developers customizing existing ROM.
However changing ROMs needs root privilege. Your phone need to be rooted before changing ROM
Eg for Custom ROMs : CyanogenMod, Hyperion, Jellyblast etc
Eg for OS : Gingerbread, Jellybean, Kitkat (upcoming)