The eye works like a camera. The refractive surfaces of the eye, the cornea and the lens, focus (refract) light onto the retina (the film in the camera) that makes sense of the picture and send the signal to the brain. The cornea does about two thirds of the refracting work in the eye. 

If the cornea focuses light in front of the retina, the brain will perceive a blurry image. This is what happens in myopia or nearsightedness. Myopia often starts at a young age and continues to increase until stabilizing in the twenties.

Hyperopia or farsightedness results when the light is focused behind the retina.

Astigmatism also blurs the image on the retina but it does so by creating two focalpoints instead of just one as in simple myopia. The shape of the cornea often creates this type of astigmatic image. The astigmatic patient often will see certain objects better than others depending on how much and where the astigmatism is located.

To correct these refractive errors, glasses and contacts act to compensate by altering light rays entering the eye in the opposite way the refractive error alters them thus correcting the error. PRK corrects the error itself by flattening the cornea to lessen the power of the cornea, in myopia, thereby focusing the image onto the retina or by flattening certain orientations of the cornea to create a single focal point on the retina in astigmatic patients.