The cornea is a special layer on the surface of the eye that does most of the work of bending light as it enters the eye. Light must be bent properly so that it creates a sharp image. Theoretically, the eye's cornea should be perfectly round, like a slice off the surface of a ping-pong ball.
When parallel light rays pass through the cornea, they should be bent just enough to meet and focus on the retina (the light-sensitive membrane that lines the back of the eye). The lens is responsible for fine adjustments of focus.
Astigmatism is a condition in which the cornea is oval, like the bottom of a dessert spoon. This means that the light rays focus at different points in the eye. As a result, the focus is smeared so that while lines in one direction may be sharp, lines in another direction will be blurry.
We don't know what causes astigmatism, but we know it tends to run in families. Some people are born with astigmatism. Others get astigmatism later in life, sometimes after eye injury, surgery, or disease.
Everyone has a tiny degree of astigmatism - no cornea is perfectly round. It's only a problem when it interferes with vision. Astigmatism is not made worse by sitting too close to the television, squinting, or reading in low light.