From this story in the New York Times:
You can lead a dog to water, but can you make him jump? That was one of the questions at Summer Splash, a three-day event at Meyer's Tails Up Farm, a dog training center an hour northwest of Chicago. More than 150 dogs competed -- yes, competed -- by leaping into a four-foot-deep pool from a dock two feet above the water. It was part of a growing sport called dock jumping, a laid-back pastime that does not have the high profile and histrionics of better-known events like Westminster. The premise behind dock jumping is simple: dogs jump into the water, and they are judged on how well they do. In this competition, a mutt is as welcome, if not more, than a purebred.
But according to the story, it's not just all about the jump. While the "Big Air" jumps are for distance, there are other categories as well, including "extreme vertical", "speed retrieve" and the all-around "Iron Dog" title.