Astigmatism is a refractive error in the eye that causes blurred vision. This phenomenon is mainly due to a misshapen cornea, lens, or the clear front cover of the eye. It's a very common condition that frequently presents itself in conjunction with other optic conditions such as myopia (nearsightedness) and hyperopia (farsightedness).
Below is an example of the differences between someone with regularly functional eyes and someone who is afflicted by astigmatism.
Astigmatism prevents light from focusing properly on the retina, which causes blurred vision without regard to distance. The exact cause of astigmatism is unknown, but it is usually an issue presented at birth with a tendency to either decrease or worsen as a person grows and ages.
There are a plethora of remedies and surgical treatments to correct one's eyesight, such as eyeglasses, contact lenses, and laser surgery.
One more interesting fact about astigmatism is that it commonly only affects one eye; as in, those with astigmatism tend to see just a bit more clearly, even WITH other optic disorders like myopia, out of one eye than the other.