Normally, the light entering the eye is brought to a focus on the Retina.  When this focus falls in front or behind the retina, a condition called refractive errors and can be of following types.

Myopic or Shortsight
In this condition the light rays falls in front of the retina.  This happens when the eye is longer than normal.

Hypermetropia or Longsight
In hypermetropia the light rays falls behind the retina.  It happens when the eye is shorter than normal.

Astigmatism
In this condition the light rays gets focused in a line instead of a point. 
These subjects experience distortion of objects or missing lines or words while reading.

Presbyopia
This is an aging process.  As we get older the lens in the eye   loses its ability to focus near objects because of reduction in accommodative power.  As a result we  find it difficult to see near objects clearly.  This normally happens around 40 years and gradually progresses as we grow older.  It may be corrected with reading glasses.