Astigmatism occurs in an eye which is not completely round. This is one type of refractive error. Astigmatism occurs in nearly everybody to some degree. For vision problems due to astigmatism, glasses, contact lenses, and even vision correction procedures are all possible treatment options.
A person's eye is naturally shaped like a sphere. Under normal circumstances, when light enters the eye, it refracts evenly, creating a clear view of the object. However, the eye of a person with astigmatism is shaped more like a football or the back of a spoon. For this person, when light enters the eye it is refracted more in one direction than the other, allowing only part of the object to be in focus at one time. Objects at any distance can appear blurry and wavy.
Source:http://www.webmd.com/eye-health/astigmatism-eyes