Ring World is a four-piece puzzle. It is a two-piece sphere contained in a two-piece ring. The purpose is to take it apart. The puzzle can be split in half when when the ring and the sphere are properly aligned. This is a metal version. You can find a plastic version here
Watch the YouTube video
Read at the Shapeways Forum
Read more at the Non-Twisty Puzzles Forum
directly if you have any further questions.