Kayaking is usually differentiated from canoeing by the sitting position of the paddler and the number of blades on the paddle.
A kayak is defined by the International Canoe Federation as a boat where the paddler faces forward, legs in front, using a double bladed paddle. Almost all kayaks have big decks although there are more and more sit-on-top kayaks. There are now several types of kayaks throughout the world. Here is a short summary of the main types of existing kayaks.
Whitewater kayaks are, in most cases, highly maneuverable boats usually for a single paddler. They include such specialized boats as playboats and slalom kayaks. White water racers combine a fast, unstable lower hull portion with a flared upper hull portion to combine flat water racing speed with extra stability in open water.
Surf skis, are specialized narrow and long boats for racing, surfing breaking waves and surf-zone rescues. Racing kayaks are designed for speed, and usually require substantial skill to achieve stability, due to extremely narrow hulls, though downriver racing kayaks are a hybrid style with whitewater boats.
Sea kayaks are typically designed for travel by one or two paddlers on open water and in many cases trade maneuverability for seaworthiness, stability, and cargo capacity.