The three most common refractive errors that can be surgically corrected with LASIK are nearsightedness, farsightedness, and astigmatism.

Nearsightedness (Myopia):
Nearsightedness or myopia occurs due to an irregularly shaped cornea - when light enters the eye it focuses in front of the retina, rather than directly on the retina. If you are nearsighted you will see well up close, but distant objects will be fuzzy.
Correcting Nearsightedness

Farsightedness (Hyperopia):
Farsightedness or hyperopia occurs due to an irregularly shaped cornea - when light enters the eye it focuses behind the retina, rather than directly on the retina. If you are farsighted you will have trouble seeing objects up close, but will see distant objects clearly.
Correcting farsightedness

Astigmatism:
The most common of all eye disorders, astigmatism occurs when the eye is shaped more like a football than a basketball. This odd shape causes the light to focus on two points of the retina, rather than one. Astigmatism is often accompanied by either nearsightedness or farsightedness. If this is the case, LASIK will treat both refractive errors simultaneously. LASIK, PRK, and other refractive procedures correct nearsightedness, farsightedness, and astigmatism by reshaping the cornea so that the focal point is on the retina.
Correcting astigmatism

What is LASIK?
Frequently Asked Questions about LASIK
Benefits and Risk of LASIK