WASP-17b is the first planet to be discovered in a retrograde orbit: it rotates about its star in the opposite direction to the spin of the star. (See the nice BBC News write-up, or the ArXiv preprint for the more technical details).

We think planetary systems are formed from a disk of debris as the star forms, and the disk and star would rotate the same way, so most planets rotate pro-grade, in the same direction of the star. For retrograde motion, it would have to have been hit early-on by some large object (another planetary embryo, for example, being ejected from the stars 'solar system' in the game of cosmic billiards as they coalesce to form planets).

Within our solar system we see retrograde motion in Venus as it spins (on its own axis) in the opposite direction of other planets, but this is the first time we've seen a planet orbit in retrograde. Similarly, Uranus' spin axis is tilted at 89 degrees, but it orbits in a pro-grade manner; its thought that Uranus was hit by a giant object causing its tilt. As we find more exoplanets, the statistics of how many are pro-grade or retrograde will help us learn about the formation of planetary systems.

WASP-17b is a "Hot Jupiter", discovered by the Wide Area Search for Planets (WASP) consortium of UK Universities by Radial velocity methods.