This question is trickier than it looks, because most people who speak English don't really think about the difference. Both can be used to express a simple future action. The following are all equally correct: "Do you think it will rain tonight?" "Yes, I think it will." "Do you think it is going to rain tonight?" "I think it is going to rain." "Will it rain tonight?" "Look at the clouds. Yes, it is going to rain." "Is it going to rain tonight?" "Yes, it will." However, "going to do something" is usually limited to simply expressing a future intention, whereas "will" sometimes also indicates agreement: Are you going to rent a car? No, I am going to buy a car. But Will you please do me a favor? Yes, I will. Finally, "will" can express more determination: "I am going to go to the circus, would you like to come along?" "I will join the circus, and there is nothing you can do to stop me." There is a long explanation at the following link, but I think perhaps the distinctions it tries to make are too rigid: http://www.differencebetween.net/language/grammar-language/difference-between-going-to-and-will/