The most obvious difference is noticeable at first glance -- the shapes of the boards are drastically different. A skateboard is a more familiar shape to the general masses, long with a rounded front and, in the case of a trick board, rounded in the back as well, with elevated lips. A RipStik instead consists of two separate boards for standing on, connected by a center rod. Rather than two wheels in front and two in back, the RipStik has only two wheels total, one centered on each of the two boards.
One of the more interesting differences that a RipStik brings to the table is the means of self-propulsion. With a skateboard, a rider must push off the ground with one foot in order to provide forward motion. A RipStik, on the other hand, is able to provide its own motion through its unique design. By alternating pressure on his back heel and front toes, then back toes and front heel, a rider can move forward on the RipStik in a weaving fashion.
Another key difference to note when selecting a board is the learning curve. A skateboard is relatively easy to learn to ride around on. The wider board and more steady platform on which to stand, make for a much easier ride. The RipStik takes a little getting used to, as mastering the swivel motion which provides your propulsion will take longer than simply learning to stand on a skateboard in a proper stance and push off at a controllable pace.
A noticeable advantage to riding a skateboard is the increased ability to perform grinding tricks. A grind is performed by sliding the bottom of the board along a flat surface, sliding as opposed to rolling on wheels. The nose and tail of a skateboard are able to be used for grinding, as are the trucks which connect the front wheels or the back wheels. The two-wheel design of the RipStik, as well as the center rod design make grinding on a RipStik significantly more difficult.
Both types of boards can be used to perform jumping tricks, and the RipStik can be used to perform most any trick that a skateboard would be used to perform. The RipStik is a slightly more difficult board on which to begin learning tricks. The good news is that for somebody attempting to make the switch from riding a skateboard to riding a RipStik, many tricks are performed with very similar motions, altered only slightly to accommodate the new board, making the learning curve shallower for experience skaters.