One difference that is useful to keep in mind is that "will happen" is purely an imagined prediction. You can be very sure about it of course, but it's still in your head. When we use "going to happen", we are predicting a future action based on watching present action. If a car is out of control and speeding towards a tree, we can definitely say "It's going to crash." Extra note: "Could you tell me what the difference is?" is the correct way to write your sentence. The question-part is already shown by "Could you...", so the rest of the sentence follows a regular word order.

The word "will" expresses simple future tense. Something is definitely going to happen. "Going to" is often used to express future tense when a decision has been made beforehand. The expressions are often used interchangeably.