One type of prewriting is called brainstorming. In this type of prewriting activity, one simply writes down -- or brainstorms -- everything that comes to mind regarding a given topic. Anything, even if it doesn't seem important, should be written down during a brainstorming session.
Another type of prewriting is called mapping or clustering. This is a useful activity to do after a brainstorming session. Here, the writer will group together similar ideas. At this point, main ideas should start to come into focus.