Will / won't (will not)(or may/might)
1. Promises and hopes for the future.
2. Predictions, using "I (don't) think...", or future facts we are certain about.
3. Together with 'probably', for future things we are less certain about.
Going to
1. Intentions
2. Predictions about the future based on evidence
Watch out; quite similar to Present Continuous, but more about the intention!