SPF stands for Sun Protection Factor. It is a measure of UVB protection and ranges from 1 to 90.
How SPF is DeterminedThe actual SPF number is determined experimentally indoors by exposing people to a light spectrum similar to noontime sun. Some people wear sunscreen and others do not. The amount of light that causes redness in sunscreen-protected skin, divided by the amount of light that causes redness in unprotected skin is the SPF.
Practical SPFHere is an easy way to remember what information SPF gives you (but not a suggestion to take sun exposure lightly):
A sunscreen with an SPF of X allows you to stay out in the sun X times longer without burning.