Andrew Conway has provided us with the following quick introduction to the subject.
The word diabolo either means "two balls", "throw across" or "yo-yo on steroids".
There was a fairly good thread on which diabolo to buy for various purposes a few months ago. To summarize:
Large diabolos are better than small ones for everything except high tosses. If you can, try several diabolos from the same manufacturer before purchasing, and choose the one that precesses the least.
The Babache "Diabolo Follies" I and II video tapes (available from Juggling Capitol in the US) are excellent. The first tape teaches beginning to advanced single diabolo tricks, and the second tape is mostly two diabolo stuff.
The following assumes you are right handed. To start a diabolo, use the string to roll it across the floor from right to left, and lift it off the ground as it is at the left hand end of your reach. Increase the spin by using the right handstick to jerk up and go down slowly. The motion of the stick comes from the *wrist*, not the *elbow*. (This is the most common mistake beginners make.) Relax the left arm, so the left handstick is pulled up and down taking up the slack.
There are several good diabolo books on the market, including those by Brendan Brolly and Todd Strong.