It's pretty well known that hair is a bunch of dead cells. Here's a little more than that:
Your hair is made of a hair fiber surrounded by debris excreted by your scalp, and a layer of sebum secreted by your glands. There are three types of hair on your body.
Lanugo — These are the first hairs that exist on your body. They exist mostly on a fetus, and cover the whole body.
Vellus — Vellus hairs grow on the 'hairless' areas of the body. They also show up in areas that are in the balding stage, and have only these fine hairs left before the area goes completely bald, or in areas that are gradually returning to the natural hair growth cycle.
Terminal — These hairs are what grow in the normally hairy areas of our bodies, such as the scalp, eyebrows, and eyelashes.