We are working on a substantial article on focus groups and how to get more value out of them. In the meantime, here is a source for general info on "How Focus Groups Work."
And, if you want to take great notes at your next focus groups, pick up a livescribe pen at Best Buy or online. It actually records the audio of the groups as you take notes and you can go back to where your wrote down that favorite quote and listen to the group at that point by tapping on your notes!
And, as epert researchers will tell you. If you want to get projectible information out of your focus groups...do a telephone survey :)