What constitutes cheating varies from one person to the next. However for the most part cheating is an act or behavior that it is usually done in "secret". More often than not at some point it requires lying directly or by omission. By definition the act of cheating is "betraying" another or breaking a promise/rule made to someone else to conduct yourself a certain way.
Cheating is something a person would not do if their mate were sitting next to them. Naturally this leaves the door wide open for everything from flirting, web cam sex, emotional cheating, having a "work spouse" you confide in, lap dances, adult websites & chat rooms, on up to physical/sexual contact. Only the person who is hurt can explain why they felt cheated on.
It's important to discuss what constitutes cheating with your mate before any potential boundaries are crossed. Awhile back I wrote a hub that deals with the issue of "assuming" monogamy.