Attached Link: http://webdesign.about.com/library/weekly/aa090501a.htm

Go to Image | Extract. Extract came into being uh.. in version 5 I think, but in version 6 Adobe beefed it up considerably. I think it does a much, much better job now. The concept is like this: You outline what you want to extract using a paintbrush tool provided within the extract plugin. You then "fill" in what you want to keep with the bucket. Photoshop automagically figures out what you want to keep and what you want to trash based on your outline. There is a decent tutorial on the basics at the link I attached.