As detailed on our Eye Health page, visible light is focused onto the back of your eye (onto the retina) through the transparent elements at the front of your eye - your cornea and your crystalline lens. The lens refracts the light onto the relevant parts of your retina and this information is fed into your optic nerve to allow your brain to create the images that you see.
However, all of this depends on the light reaching the correct parts of the retina with the correct clarity and brightness, and for this your cornea and lens to be perfectly round. Astigmatism occurs when the cornea and / or lens are of a different curvature to normal and thus refract visible light onto different parts of the retina. Your eyes are designed to focus light at certain distances through a round cornea; with an astigmatism only part of this light can be properly focused. This results in slightly distorted vision in which only a part of your vision will be in focus at any one time.
Because this different curve of the lens and cornea effectively alters the shape of your eye, Astigmatism is sometimes described as being more ‘rugby ball’ shaped instead of the normal ‘football’ shape. Remember - astigmatism is not a disease, nor does it mean that you have ‘bad eyes’. All it basically means is that you have a variation in the shape of your cornea to normal. It can be corrected with prescription glasses or contact lenses designed specifically for astigmatism.