A good example of an application of a daemon thread would be some type of background clean-up operation. If clean-up is only operating on the Java heap, then there is no need to delay JVM shut-down while a clean-up operation completes. If you put the clean-up operation in a Daemon thread, it won't delay shut-down.
Betty Rubble? Well, I would go with Betty... but I'd be thinking of Wilma.
Peter is right - learn to love the JavaDoc! He also gave a much nicer explanation than the JavaDoc which has some legalese double-negative convoluted wording about daemon. I always have to read it about 5 times to parse it.
A good question is never answered. It is not a bolt to be tightened into place but a seed to be planted and to bear more seed toward the hope of greening the landscape of the idea. John Ciardi