Brainstorming is a method or tool in which a project team sits and talks about all kind of ideas to solve a specific problem. It is a great platform to develop and motivate a team.
There are lot of great articles flowing in the cloud which talks about what it is, how to do it and other stuff on this. I believe a picture tells a better story then text. So here is my pictorial representation of Brainstorming.
Brainstorming is such a tool that you can use in any phase of Project Management but mainly this is used in Planning phase.