Question

Which is a main difference between brainstorming and freewriting?

Answer 1

Brain storming is when you get ideas for writing or anything to that matter. 
and free writing is usually when you just write about whatever you want

Answer 2

Brainstorming: Your basically thinking of ideas. writing down ideas so later you can put those ideas to use.
Freewriting: Basically freestyle. Just write whatever comes to mind, the second it does.