The HotSpot JVM contains several garbage collectors. They are:

1. Serial GC : The serial collector is the default for client style machines in Java SE 5 and 6. With the serial collector, both minor and major garbage collections are done serially (using a single virtual CPU).  In addition, it uses a mark-compact collection method.
2. Parallel GC : The parallel garbage collector uses multiple threads to perform the young generation garbage collection. By default on a host with N CPUs, the parallel garbage collector uses N garbage collector threads in the collection.
3. CMS Collector : The Concurrent Mark Sweep (CMS) collector collects the tenured generation. It attempts to minimize the pauses due to garbage collection by doing most of the garbage collection work concurrently with the application threads.
4. G1 Garbage Collector : The Garbage First or G1 garbage collector is available in Java 7 and is designed to be the long term replacement for the CMS collector. The G1 collector is a parallel, concurrent, and incrementally compacting low-pause garbage collector that has quite a different layout from the other garbage collectors described above.