This week's book giveaway is in the General Computing forum. We're giving away four copies of Arduino in Action and have Martin Evans, Joshua Noble, and Jordan Hochenbaum on-line! See this thread for details.
It's just a flag in the Thread object. The JVM will continue to run only as long as non-daemon threads exist. As soon as the only running threads have the daemon property set, the JVM will exit. That's it -- nothing more.