They are algorithms for the young generation garbage collection.
The second one (UseParNewGC) gets activated automatically with the concurrent tenured generation garbage collection (see Java Concurrent and Parallel GC) but, is there a difference between the two parallel algorithms?
After a lot of searching, the best explanation I've found is from Java Performance Tuning website in Question of the month: 1.4.1 Garbage collection algorithms, January 29th, 2003
Young generation garbage collection algorithms
The (original) copying collector (Enabled by default). When this collector kicks in, all application threads are stopped, and the copying collection proceeds using one thread (which means only one CPU even if on a multi-CPU machine). This is known as a stop-the-world collection, because basically the JVM pauses everything else until the collection is completed.
The parallel copying collector (Enabled using -XX:+UseParNewGC). Like the original copying collector, this is a stop-the-world collector. However this collector parallelizes the copying collection over multiple threads, which is more efficient than the original single-thread copying collector for multi-CPU machines (though not for single-CPU machines). This algorithm potentially speeds up young generation collection by a factor equal to the number of CPUs available, when compared to the original singly-threaded copying collector.
The parallel scavenge collector (Enabled using -XX:UseParallelGC). This is like the previous parallel copying collector, but the algorithm is tuned for gigabyte heaps (over 10GB) on multi-CPU machines. This collection algorithm is designed to maximize throughput while minimizing pauses. It has an optional adaptive tuning policy which will automatically resize heap spaces. If you use this collector, you can only use the the original mark-sweep collector in the old generation (i.e. the newer old generation concurrent collector cannot work with this young generation collector).
From this information, it seems the main difference (apart from CMS cooperation) is that UseParallelGC supports ergonomics while UseParNewGC doesn't.