Blankets serve many different purposes. Mostly they provide a warm, covering bedding. But they are also good to picnic on or hide under. Blankets can be a warm wrap to take to a ball game or pad the bleachers. And don't forget the toddler's security blanket.
Generally blankets are made of thick material with edging -- often a looser weave and thicker fibers than other cloth in order to hold more air and provide better insulation. The weave may also hold other, thick fibers in place; knitted and crocheted blankets also provide the thickness and desired warmth.
Wool is popular because of its warmth. But wool has the disadvantages that some people find it scratchy or have allergies to it.
The weave of a blanket tends to be loose. The air held in the loose weave helps provide insulation. In addition to a loose woven cloth, it may also be a fleece or flannel style.
- Wikipedia's article on blanket