Will vs. Going to
Will and going to are the two most common ways of expressing the future. While they are similar in many aspects, there are three differences:
1) Predictions
Predictions expressed by "will" are based on your intuition and experience.
Predictions expressed by "going to" are based on what's happening at the present moment.
2)
"Going to" usually refers to the near future. While will doesn't refer to any specific moment (it may sometimes concern very far future).
3) Intention
This is a subtle difference but it is noticable: if someone uses the "going to" form, the person is often talking about his or her intentions. In other words, there is an ephasis on the intention of doing something.