Prescription Explained

 At the end of your eye test you will be given a prescription which will look something like the example below.

A prescription is usually valid for two years, but your optometrist may recommend that you have your eyes tested more frequently than this, depending on your particular circumstances.
Remember, an eye test does not only check that your prescription is right for you; it is also a check on the health of your eyes and visual system. Many eye diseases can go unnoticed until it is too late to treat them effectively so it is important to have regular eye tests even if you feel you can still see well.

What does it all mean?

The "sph" box means sphere which represents the amount of long or short sight that is present.  The larger the number the stronger the lens required, and therefore the thicker the spectacle lens will be.  Plus lenses are used to correct long sight and minus lenses are used to correct short sight.
The "cyl" box for cylinder represents the amount of astigmatism that is present.  Astigmatism is caused when the eye is not completely spherical (like a football), its shaped more like a rugby ball.  This causes the vision to be distorted for both distance and near objects.  The cylinder may be plus or minus regardless of whether the sphere is positive or negative.
The "axis" box represents the orientation of the cylinder (from 0-180 degrees) and is the angle at which the lens is set into the frame.
The "prism" is the correction needed (if any) to align the eyes, so that they are looking straight and working well together.  A prism is a lens that bends the path of light without altering its focus.   
The "add” box is the amount of correction that must be added to your prescription