What are refractive errors of the eye?
Myopia - Myopia focuses light in front of the retina.
Myopia, or nearsightedness, occurs when the eye has too much focusing power. Therefore, objects at a distance are blurry, whereas objects that are close can be clearly seen (unless astigmatism is present). Typically, the myopic eye is slightly longer or has a steeper cornea compared to the normal eye.  

Hyperopia - Hyperopia focuses light behind the retina
Hyperopia, or farsightedness, occurs when the eye lacks sufficient focusing power. This usually causes objects at a distance to appear blurry, and near objects are seen even less clearly. Typically, the farsighted eye is slightly shorter or has a flatter cornea than the normal

Astigmatism - Astigmatism focuses light in an oblique fashion, so no true image
forms on the retina
Astigmatism is a condition in which light rays in different orientations are brought to different points of focus. A useful analogy is the comparison of a basketball to a football. The surface of a basketball has equal curvature in all directions, whereas a football is flatter in one orientation and steeper in the other. The surface of the football represents the situation that is present in the cornea when the eye has astigmatism. This surface focuses light obliquely, so no true image forms on the retina.

Usually, the astigmatism occurs in the cornea. This often can be corrected with glasses or contact lenses, or in many instances, treated with refractive surgery.

Presbyopia - Presbyopia focuses light behind the retina at near distances only. This is because the eye doesn't have the strength to "autofocus" at reading distances
Presbyopia is a natural condition that typically becomes noticeable around age 45. In children and young adults, the crystalline lens inside the eye can easily look at a distant object and then increase its focus to look at a near object. As we get older, the lens inside of the eye loses this capacity to focus in and out. As this occurs, individuals find that they need to hold reading material at greater distances from them. Ultimately, reading glasses are needed, typically by the mid-40s.