1) Whitewater kayaks: This is what most people think of when they think kayak. Tearing down some rapids, water splashing everywhere and a lot of speed and danger. These are shorter to increase the maneuverability of the kayak around rocks and in fast moving water. They may have rounded bottoms and more upturned ends to deal with the waves. They are hard to paddle in a straight line and are not recommended for casual kayaking. You can buy paddle board online from many sources.
2) Downriver kayaks: These are specially designed to travel quickly and smoothly through the water. These are normally used for racing as they are very long and narrow which makes them tipsy and not very much suited for a novice kayaker. They are very long and straight, which makes them go in a straight line easily; however, turning is more difficult.
When you buy a kayak, you must consider how you will use it. If you are going for easy little day trips in a calm river or braving the roar of the rapids. You will also need to consider if you will be turning a lot, how fast you need the boat to be and the boats volume. The volume relates to how much stuff you can store in your kayak when you take it out. The more stuff you need to bring, the more volume it needs.
Lastly, the shape of the boat is important to consider. You will want to talk to your kayak store specialist and let them know exactly how you intend to use your kayak and how skilled you are. You will also need to get gear for yourself including a helmet, lifejacket, clothing, first aid gear and waterproof bags for any electronic devices you may need.