I am a Android greenhorn, and seems like there are a lot of jargon that float around.
There are several reasons to be looking for a ROM:
Upgrading or downgrading your stock ROM
Looking for firmware from a different region/language
Flashing a custom ROM for different features, removing ...
Isn't Android a common kernel that is used across all devices? For example, CentOS will install on Dell, HP, and a variety of other hardware. Sure there are different modules, but it's still CentOS ...