We use "will" when we decide to do something at the time of speaking, while "going to" is used when at the time of speaking, we have already decided to do something. 

So if you are talking to a friend and the idea of organizing a party comes to your mind, you can say: Let's have a party. We'll invite all our friends.

If a few minutes later you talk to another friend, you say: My friend and I have just decided to organize a party. We are going to invite all our friends.

If someone tells you that your mutual friend is in hospital, and this information is new for you, you say: "I didn't know that. I'll go and visit him." If you already know about this, you say: "I know. I'm going to visit him this afternoon."

Hope this helps.

You can search online using these keywords: <em>difference will going to</em>

Be assured that this question has been asked and answered literally thousands of times before. Why rewrite what has already been written? :) 

The simple answer:  "going to" Planned

"will"  not planned

http://www.italki.com/discussion/88648