Question: Recently I heard Kirk Pepperdine speak about changing garbage collectors for better performance -- but what exactly does that mean and what makes one garbage collector better or different than the other?

Answer: 
Some collectors are better for throughput, others are better for response time. The difference is usually in how the collector chooses to pause the application. Some such as CMS use mutiple passes to triage the garbage before stopping the application. This triage can happen in a background thread while the application is running, and thus not interfere with your application as much as one that "stops the world" to do a GC.

**Edit**

Check out [this document by sun](http://java.sun.com/developer/technicalArticles/Programming/turbo/). Also, about half way down there is a nice image showing the default mark-compact collector against the CMS collector. A picture is worth a thousand words, but the article is a good read too ;) Also worth reading is all the documents on the new G1 collector.
The basic problem is that the way that Java program sees memory (you call "new MyObject" and there it is, and when you are done with it you just forget about it) does not map very well to the underlying operating system and hardware.

The job of the garbage collector is to identify those memory areas which are not in use by an object, and "melt" them together to give a LARGE memory area from where new objects can be allocated. This is very vaguely worded in the Java specification HOW this is done, most likely in order to provide maximum flexibility for the designers of this important task. 

Several approaches exist, with advantages and disadvantages. What you usually want is a garbage collector that can keep up in the background with the rate of objects being abandoned, as the only way for it to catch up is to stop the program while catching up. That gives really bad user experiences.

A typical trend for Java objects is that either they live for a very short time (current block or method) or a very long time. Modern garbage collectors deal with this by having multiple pools so that young objects are treated differnetly than old objects.