How your eyes work: 
To see clearly, the cornea and the lens must bend - or refract - light rays so they focus on the retina - a layer of light-sensing cells that line the back of the eye. The retina converts the light rays into impulses that are sent to the brain, where they are recognized as images. If the light rays don't focus on the retina, the image you see is blurry. This is called a refractive error. Glasses, contacts and refractive surgery attempt to reduce these errors by making light rays focus on the retina. 

Refractive errors are caused by an imperfectly shaped eyeball, cornea or lens, and are of three basic types:

Myopia: The optical power of the eye is too strong, and the light rays from distant object are focused to a point in front of the retina.
 

 Myopia

Hyperopia: The optical power of the eye is too weak, and the light rays from distant object are focused to a point behind the retina. Picture
 

Hyperopia

Astigmatism: The anterior refracting surface of the eye is slightly elliptical rather than spherical. This causes light rays from a distant object to come into focus at two discrete points in the eye.
 

Astigmatism

Presbyopia: The ability to focus at near is deteriorated according to ages. If you are more than 40, you need to wear reading glasses.

How to correct refractive error