Chapter 10. Details
Daemon Thread Groups
Calling Native Code
A Few Assorted Methods
The Effect of Using a JIT
APIs Used in this Chapter
The Class java.lang.Thread
The Class java.lang.ThreadGroup
In which a number of minor details are covered.
A thread group is a group of threads, or more precisely, a group of threads (possibly empty) and other thread groups (also possibly empty). The raison d'être for thread groups is security. Java needs some method of allowing you to download untrusted foreign code and run it without it being able to affect the rest of your program.
The idea was that you run the foreign code in its own thread group and tell that thread group that it is not allowed ...