Brainstorming is a process of generating idea that encourages alternatives while withholding any criticism of those alternatives. In a brainstorming session, people gather in a room (or teleconference) and the group leader states the problem clearly so that all participants can understand it. Then the participants suggest as many alternatives as they can in a given time. No criticism is allowed during the session, and all alternatives are recorded to be discussed and analyzed later.
Brainstorming is only a process of generating ideas. The nominal group technique, which is the next step that should be taken, is the one that helps the group to arrive at a preferred solution.