A kayak is a small boat with a deck that is propelled by a two blade paddle by someone sitting, unlike a canoe which is open and propelled by a single blade paddle by someone kneeling. Kayaks are designed for one person or two and sometimes more. Kayaking is a growing sport, and various types of kayaks are used for different kinds of water, including whitewater, rapids and waterfall kayaking, ocean or sea kayaking, flat-water stream kayaking, and lake and pond kayaking.
Buying a kayak really depends a lot on what you're going to do with it. You need to make sure that you buy the proper kayak for your uses. There are many different kinds of kayaks, and each have areas that they are designed to excel in. Some may have sport ter manuverability, but lack in speed. Others the opposite way around.
The recreational kayak is stable and easy to paddle. These boats are designed for lakes, bays and slow moving water. Good for beginners, they are short and wide and easy to turn, but more difficult to paddle in a straight line. Recreational kayaks come in different sizes for comfort and often have storage areas for daytrip equipment.
Touring or sea kayaks are long and stable, these boats are good for extended trips. They track well in a straight line, but are harder to turn than shorter boats. Varying models have more of less capacity to store gear. For day trips, you may want to choose a lighter touring boat with less storage and more maneuverability.
Whitewater kayaks are agile but less stable than other kayaks. These boats are designed to fit tight around the body, to make quick turns and be easy to maneuver. They usually have rounded bottoms or flat hulls for negotiating rapids. They are difficult to track in a straight line.
The downriver kayak is often used for racing by more advanced kayakers, these long and narrow boats are designed for speed. Their straight keel makes them easier to track in a straight line, but difficult to turn, and they are easy to tip.