"Will" is a very important, basic word in English. It's original meaning was "wish" or "desire". Now we use it in many ways. The word has become an important part of English grammar.

It is used in these main ways:

1. to talk about the future;
2. to express certainty;
3. to talk about decisions and willingness;
4. to make requests and orders;
5. to make an offer;
6. to be indirect and polite; and
7. to talk about habits.

"Be going to" is used like an auxiliary verb. We use "be going to" just like other auxiliary verbs (e.g. "can" or "should"):

1. "I can go";
2. "I should go";
3. "I am going to go".

When we use auxiliary verbs, we change the meaning of the sentence a little. In the examples above, using different words changes the sentence from being about ability (1), to obligation (2) and then to a plan (3):