SPF refers to Sun Protection Factor and applies only to UVB radiation. It is a multiple of how much time you can stay in the sun without burning your skin. For example, if you burn without any sunscreen in 10 minutes, a sunscreen with an SPF 15 would allow you to stay in the sun for 150 minutes (2 hours and 30 minutes) without burning. (SPF15 x 10 minutes = 150 minutes)

SPF describes the increased allowable time of sun exposure before your particular skin type burns. An SPF value is assigned by manufacturers of sun protecting cosmetics for items such as sunscreens, creams, lipsticks, cosmetic milks, lotions, etc. For example, SPF4 means that you can expose yourself to the sun four times longer without burning.