Brainstorming is a quick method to explore a lot of ideas early in a project. This may include white board sessions with a group and product idea boards which map an entire user experience.
At this point in a project, many ideas are explored which can be vetted, criticized, edited and built upon. Many times we will take certain portions of one project and combine them with ideas and portions of other projects.
The goal at the end of a brainstorming session is to provide a direction for further design development with an interactive prototype.