By definition, a kayak is a decked boat which is paddled from a seated position using a two-bladed paddle. If you have perused our site then you know that this definition leaves a lot of room for designers to work in. Kayaks have an much undeserved reputation for being tippy, however.
People look at a wide canoe and a narrow kayak and assume that the canoe is much more stable.
The difference is in the center of gravity in both boats. A canoe has to be wider because the paddler is up higher in the boat. In a kayak you also have the advantage of your paddle. Just like a tightrope walker you have a pole held across your body that increases your balance. So in practice, kayaks aren't any more tippy than canoes or any other small boats.