The Golden Snitch, commonly known just as the Snitch, is a small golden ball the approximate size of a walnut (roughly an inch in diameter). Unlike the other balls, the Snitch is winged and enchanted to fly around the pitch, avoiding capture while remaining within the boundaries of the playing area. Each team has a designated Seeker, whose task is purely to capture the Snitch. It is also explained in Harry Potter and the Deathly Hallows that the Snitch has a "flesh memory" for the first person to touch it in order to resolve disputed catches.
(Explore, 1.16.2008--Thanks friends!)