Browse around this site for pictures. Monohulls have one of these.
Catamarans have two.
A useful side effect is that the hulls can be made very narrow without making the boat too tippy and difficult to balance. This means the hulls can slice through the water easily, and this is part of the reason why they're so fast. The Dart18 is not the only catamaran available, but there a number of reasons why many people consider it to be the best. Compared to monohulls, you get a lot of boat for your money. A new boat costs less than 6000, ready to sail.