There are lots of things that could be it and not exclusively to:

1. Defective IAC Valve.
2. Bad Gas.
3. Clogged Fuel Filter.
4. Clogged Fuel Injectors.
5. No Current to Spark Plugs.
6. Clogged Air filter.
7. Dirty throttle body.

There could be other things that it could be but it is hard to fully diagnose without taking a look at it. I remember that if the IAC valve is bad, the car will die when you take your foot off the gas.