The heap is split into several different sections, called generations.
As objects survive more garbage collections, they are promoted into different generations. The older generations are not garbage collected as often. Because these objects have already proven to be longer lived, they are less likely to be garbage collected.
1. Eden Space
2. Survivor Space
3. Tenured Generation
4. Permanent Generation, or PermGen.
When objects are first constructed, they are allocated in the Eden Space. If they survive a garbage collection, they are promoted to Survivor Space, and should they live long enough there, they are allocated to the Tenured Generation. This generation is garbage collected much less frequently.
There is also a fourth generation, called the Permanent Generation, or PermGen. The objects that reside here are not eligible to be garbage collected, and usually contain an immutable state necessary for the JVM to run, such as class definitions and the String constant pool.
PermGen space is planned to be removed from Java 8, and will be replaced with a new space called Metaspace, which will be held in native memory.
Using the PermGen Space
For most applications, the PermGen area contains traditional class definitions, String constants, and not much else. Newer languages running on the JVM, such as Groovy, have the capability to create dynamic class definitions, and when used under load, this can fill up the PermGen space easily. You must be careful when creating many dynamic class definitions; and you may need to tweak the default memory allocation for PermGen space.