This week's book giveaway is in the OCPJP forum. We're giving away four copies of OCA/OCP Java SE 7 Programmer I & II Study Guide and have Kathy Sierra & Bert Bates on-line! See this thread for details.
"Worker threads", "system threads" and other kinds of threads are not really totally different kinds of threads. There isn't a fixed list of types of threads. What "type" you'd assign to a thread depends on what you are using the thread for. A thread that does work in the background you could call a "worker thread", for example.
You can't say what type of thread a thread is that you create yourself by implementing the Runnable interface.