Calories burned > calories ingested = lose fat
Calories burned < calories ingested = gain fat
Calories burned = calories ingested = no loss/gain
Simplest concept of losing weight. Period.
It really doesn't get any more simpler than this ^. Cardio is the best. More specifically, sprinting. That's where it really hits the stomach area. Yeah, go for time (be able to run for an hour straight at the same pace non-stop) rather than just setting a goal to 5 miles and trying to reach it in 30 minutes if you were just going to run.