Basics - cv joints are the flex joints (similar in function to u-joints) that are at the outer ends of the front drive axle. They transmit power and allow the front wheels to turn. There is a rubber boot over each, and it may split open, allowing grease to escape and all kinds of crud in, contaminating the joint and causing premature failure.
I found this old thread which primarily discusses replacing the boot, but also mentions replacing the half-shaft (assembly includes the cv joint). If you need more info, and don't get enough in this thread, try the search feature, and maybe try to contact JEEPTECHNICIAN, a member here who posted in this thread http://www.jeepforum.com/forum/f13/cv-driveshaft-boot-torn-42526/