In testing vision, eye doctors measure visual acuity with The Snellen chart (Snellen Chart Image). This tool, which most if not all of you have seen in your eye doctor’s office, begins with a very large letter at the top and has progressively smaller letters moving downwards. "20/20" means that the person being tested can see the same letters on the chart, viewed from 20 feet, as a theoretically normal eye can see from 20 feet. "20/80" means the eye sees at 20 feet what a normal eye sees at 80 feet.
Optical prescriptions are written in diopters. A typical prescription is illustrated here (Optical Prescription Image) to specify left / right eye (O.D./O.S.), sphere power (SPHERE), cylinder power (CYL), axis of cylindrical correction (AXIS), and the extra plus power (ADD) added for bifocals or progressive lenses (no line) at the bottom of glasses.