1. Is a business that makes money. 2. See point 1. above
Everything else is considered either art (as in artist) or hobby.
Examples of non-startups:
1. A iPhone developer with 1 app, no revenue 2. A web development shop with no customers - or no significant growth potential (that's a hobby). Especially if you work for someone else. 3. You make no money - this is a "non-profit organization" or "charity" - or a "research institute". You often have to do other jobs on the side to eat, live.
Example of a startup:
37 signals build software for internal purposes (eg: Basecamp). This would initially lead you to believe this is a not a startup, but then the started charging for it on a mass level, providing good for other people, etc. This is a startup. Once you mature the revenue, you are no longer a startup.