Myopia, or nearsightedness
Myopia, or nearsightedness, occurs because your eye shape is too long and / or you have an excessively steep cornea. Light entering your eyes does not focus on the retina as it should, but instead focuses on images at a point in front of the retina. The result of nearsightedness is that your distant objects appear blurry, while near objects appear clear.

Hyperopia, or farsightedness
Hyperopia is also called farsightedness and occurs because either your eye shape is too short or you have an excessively flat cornea. In farsightedness, light entering your eye focuses on images at a point behind the retina. The result of farsightedness is that near objects you are seeing appear blurry, while distant objects appear clear. In some cases, hyperopia may cause blurriness at both distance and near.

Astigmatism
Astigmatism occurs because you have a cornea that is not spherical as is a normal eye, but your cornea is typically shaped more like a football (rather than a basketball). The result of astigmatism is that the objects you are viewing are not focused into a single image and vision is distorted or blurry. Astigmatism can be present alone or in addition to nearsightedness or farsightedness.

Presbyopia
Presbyopia is the inability to focus on objects up close.  This occurs normally with age and usually becomes apparent to people in their forties. Presbyopia results from a change within the eye in which the internal lens loses its ability to focus on close-up objects.