We receive more information about our surroundings through our visual sense than through any of our other senses. We are able to interpret the shapes, colors, and dimensions of objects by the light rays they give off. When light rays enter the eye, they are bent, or refracted, by the cornea (the clear tissue on the front of the eye) and the lens (the transparent structure inside the eye) so that they are focused directly on the retina (the tissue at the back of the eye where visual sensory receptors are located). The retina then transmits these images to the brain, where they are processed.
When the light rays are focused perfectly on the retina, the result is 20/20, or normal, vision.
When light rays are not focused, or refracted, precisely onto the retina, a "refractive error" results.
Myopia is a refractive error that can occur when the curve of the cornea is too steep or the eye is too long in relationship to its corneal curvature. In this case, light rays focus in front of the retina instead of on its surface, which results in blurry distance vision.
Hyperopia can occur if the cornea is too flat or the eye is too short. This results in light rays focusing at a hypothetical point beyond the retina, causing vision blurry near vision.
Astigmatism is a refractive error that results from the cornea being more curved in one direction than the other (like a football). This causes light rays to focus in more than one point on the retina, which makes vision blurry and often distorted, at all distances. Astigmatism often occurs in combination with myopia and hyperopia. (Photo courtesy of EyeSmart: http://www.geteyesmart.org/eyesmart/diseases/astigmatism/index.cfm)
Presbyopia is the normal process of having more difficulty focusing at near distance that occurs with advancing age. In order for our eyes to focus on objects at a near distance, the lens in the eye needs to change shape. However, as we approach middle age, the lens begins to thicken and loses its flexibility. This results in blurred vision at near distance.
Glasses or contact lenses often help correct these refractive errors. LASIK and other vision correction procedures are designed to help eliminate or reduce the need for eyeglasses or contact lenses.