In order to see clearly, rays of light pass through the cornea, pupil and lens of the eye.  The image will be focused directly on the retina.

In a nearsighted person the cornea is too steep or the eye is too long and the focal point is in front of the retina resulting in blurred distance vision.

For a farsighted person the focus of the eye is behind the retina. People who are farsighted can have trouble seeing at distance and near.

Astigmatism occurs when the cornea is not completely round but shaped more like a football.  Light that enters the eye focuses on more than one spot on the retina resulting in blurring of vision. Astigmatism can occur with nearsightedness or farsightedness.