will and going to

What it is the difference between 'will' and 'going to'?

In many examples native speakers use these interchangeably.
There are rules, but they are often ignored especially in spoken English, and the meaning is not changed.

Will + infinitive 
A decision at the moment of speaking:
Jack: There's no rice left.
Sarah: Really? In that case, I'll go and get some. 
(Sarah didn't know the rice was finished and decided at that moment she would go and get some)

Be going to + infinitive
A decision before the moment of speaking:
Jack: There's no rice left.
Sarah: I know. I'm going to go and get some after I have finished cleaning.
(Sarah knew the rice was gone and had already planned to go to the market to buy some)

A prediction based on opinion:
I think it will rain this afternoon. 
(I am not sure why, I just have a feeling it will rain)

A prediction based on something we can see (or hear) now:
It is going to rain this afternoon. The clouds are already big and black.
(I know it will rain as the rain clouds are already heavy and overhead)