Wikipedia page: http://en.wikipedia.org/wiki/Rubik%27s_Clock
The Rubik's Clock has two sides each with nine clock faces which have a single hand. When all the clock hands on both sides are pointing up to the twelve position the puzzle is solved. The clock faces can be rotated in place by a combination of four wheels (on the corners) and four pins (on the faces). The wheels each move a group of four faces: the faces that surround the pin when it is raised...
Hmmm, this is harder to explain than I thought! I shall seek out a better description.
At time of writing, 10/10/10, I have an official WCA average of 25.94 seconds which gives me a national ranking of 8th and a world ranking of 238. Not bad for a puzzle I learned to solve in an evening. I use Stefan Pochmann's Clock Speedsolving method which seems to be very popular.
Almost everybody in the speedsolving community uses original Rubik's brand clocks that were manufactured in the 1980s. You can pick them up on ebay for about a fiver or less if you're lucky. I have owned quite a few including one that was assembled incorrectly such that it was impossible to solve!