What is it?
Brainstorming is an group based idea generation technique which helps to reveal many solution alternatives in a short period of time.
How can I use it?
Focus on the subject, contribute to the discussion. One of you should take notes. Be open minded, do not judge any idea, also build on that ideas. Quantity is important.
When can I use it?
When you can not define the problem or solution exactly and you need to reach new ideas immediately, it can be one of the alternative. Especially at the beginning of a new project.