Young generation collectors

Copy (enabled with -XX:+UseSerialGC)
the serial copy collector, uses one thread to copy surviving objects from Eden to Survivor spaces and between Survivor spaces until it decides they've been there long enough, at which point it copies them into the old generation.

PS Scavenge (enabled with -XX:+UseParallelGC)
the parallel scavenge collector, like the Copy collector, but uses multiple threads in parallel and has some knowledge of how the old generation is collected (essentially written to work with the serial and PS old gen collectors).

ParNew (enabled with -XX:+UseParNewGC)
the parallel copy collector, like the Copy collector, but uses multiple threads in parallel and has an internal 'callback' that allows an old generation collector to operate on the objects it collects (really written to work with the concurrent collector).

G1 Young Generation (enabled with -XX:+UseG1GC)
the garbage first collector, uses the 'Garbage First' algorithm which splits up the heap into lots of smaller spaces, but these are still separated into Eden and Survivor spaces in the young generation for G1.

Old generation collectors

MarkSweepCompact (enabled with -XX:+UseSerialGC)
the serial mark-sweep collector, the daddy of them all, uses a serial (one thread) full mark-sweep garbage collection algorithm, with optional compaction.

PS MarkSweep (enabled with -XX:+UseParallelOldGC)
the parallel scavenge mark-sweep collector, parallelised version (i.e. uses multiple threads) of the MarkSweepCompact.

ConcurrentMarkSweep (enabled with -XX:+UseConcMarkSweepGC)
the concurrent collector, a garbage collection algorithm that attempts to do most of the garbage collection work in the background without stopping application threads while it works (there are still phases where it has to stop application threads, but these phases are attempted to be kept to a minimum). Note if the concurrent collector fails to keep up with the garbage, it fails over to the serial MarkSweepCompact collector for (just) the next GC.

G1 Mixed Generation (enabled with -XX:+UseG1GC)
the garbage first collector, uses the 'Garbage First' algorithm which splits up the heap into lots of smaller spaces.