Oh dear. You probably should start here first:
http://en.wikipedia.org/wiki/Rom
To ask "What exactly is a ROM?", you should try to understand generically what a ROM is.
Try to research, the answers are out there, all over. By asking to be spoonfed, you likely won't get the response you seek. Just look around and start reading. Try to be more self-sufficient. After some resarch, then ask more specific questions. People are likely to answer more specific questions.
Good luck.