People don't use "shall" in casual speech, especially in America. It means exactly the same as "will," but the person who uses it just wants to sound smarter. A long time ago it had a purpose, and here's the way it was "supposed" to work:
There were two ways of expressing the future - a normal way and an emphatic way.
Normal way: I shall, we shall, he/she will, they will, you will
Emphatic way: I will, we will, he/she shall, they shall, you shall
But I don't think anyone really used that in speech.
I concur with JacquiD on usage of shall. Her examples number 2 and 3 are the only times I can think of where you can't replace "shall" with "will."