An optometrist will test your vision and determine the amount of refractive error in your eyes, and write a "prescription" as close as possible to that. A standard "prescription" should correct your vision so that there is no blur at 20 feet. If you can not see clearly on your corrective lenses at 20 feet, see your optometrist and make sure you know about and understand any underlying conditions you have that affect your eyesight. Remember that having refractive error does not indicate a medical condition, despite the implication in the word "prescription".
A "prescription" may look like this:
OD is oculus dextrus, or Latin for right eye. OS is oculus sinister, or Latin for left eye. BO is Base out, BI is Base in, BU is Base up and BD is Base down.
Pupillary Distance or PD, is the final set of numbers you need to be aware of. Most likely this is not listed on your "prescription" you will need to ask for it or measure it yourself.
You will note that most "prescriptions" will round to the nearest quarter diopter. This is the level of precision available in standard pre-ground lenses, which make glasses cheaper and faster to make than custom ground lenses. You may have a more precise prescription if it is written for specific lens grinding systems, or a less precise prescription if it is written for contacts.
Contacts Vs Glasses
See also Lens#Glasses or Contacts
Your "prescription" is specifically written for either contacts or glasses. Vertex Distance plays a role in effective lens strength, so the contacts which sit closer to your eye may be written with less correction than glasses. Vertex Distance is hard to measure, and contacts may act differently as the eye adapts to them, so if you have a strong lens your optometrist may need to do additional refraction tests with the contacts in and adjust your prescription slightly.