Brainstorming is simply listing all ideas put forth by a group in response to a given problem or question. In 1939, a team led by advertising executive Alex Osborn coined the term "brainstorm." According to Osborn, "Brainstorm means using the brain to storm a creative problem and to do so "in commando fashion, each stormer audaciously attacking the same objective."

Creativity is encouraged by not allowing ideas to be evaluated or discussed until everyone has run dry. Any and all ideas are considered legitimate and often the most far-fetched are the most fertile. Structured brainstorming produces numerous creative ideas about any given "central question". Done right, it taps the human brain's capacity for lateral thinking and free association.
Brainstorms help answer specific questions such as:

What opportunities face us this year?
What factors are constraining performance in Department X?
What could be causing problem Y?
What can we do to solve problem Z?

However, a brainstorm cannot help you positively identify causes of problems, rank ideas in a meaningful order, select important ideas, or check solutions.