Use definitely to describe something you don’t have any doubt about. If you are certain you're going to see your friend's band play, you can assure him that you'll definitely be there.
The adverb definitely is used to emphasize the certainty of whatever word it modifies. If you are sure you want apple pie, you could say you definitely want it. The base word definite can also mean "a certain limit," like the definite size a shelf must have to be. You wouldn't say "the carpenter definitely cuts the wood," to describe the exactness of the work; use precisely instead. AND, yes, definitely definitely has an e before the ly!