As an eyeglass-wearer, I have gone through the following experience several times: I go to the optometrist, and she tests my eyes and gives me a little piece of paper. I take this paper to the optician, who then makes my glasses. But what do all the numbers and symbols on the prescription mean? This is an attempt to explain them.

The prescription usually contains a table which looks like this:

        |  Spherical  |  Cylindrical  | Axis       |
O.D.    |             |               |            |
O.S.    |             |               |            |

In each cell of the table is written a number. Let's break down the different headings:

O.D. and O.S.
These represent the right and left eyes, respectively.
This is the spherical refractive error. Basically, whether you are near-sighted (or myopic), or far-sighted (or hyperopic). This number is measured in diopters; for example, -2.50 means that the refractive error has a degree of two and a half diopters. The - sign indicates near-sightedness; a + sign is used for far-sightedness. 0.00 indicates that no correction is needed; this is otherwise known as plano.
This is a indicator for the degree of astigmatism, also measured in diopters. Astigmatism is a misshaping of the eye that causes the image that a person sees to be blurred in one direction. As a result, the corrective lens also needs to be curved in a particular direction. This leads us to:
This number, measured in degrees between 90 and 180, indicates the direction of astigmatism.

Generally, the spherical component is considered the main correction, and the cylindrical component is considered to be "fine-tuning". Optometrists will first find out the best spherical correction, and then afterwards correct for astigmatism if necessary.