SPF stands for Sun Protective Factor. It is actually a number based on the minimal erythema dose (MED) multiplied by the SPF number. The MED number is the amount of sun exposure required to produce mild redness in the "average" skin.

Multiply your SPF number by 10 to find out how many minutes you can stay out in the sun without burning. If it takes 10 minutes to produce redness in the "average" skin (the MED number) and the SPF of your product is 15, then it takes 10 x 15 = 150 minutes of sun exposure before your skin begins to show redness.

A few frequently asked questions:

Will lots of melanin protect my skin?
Not necessarily. Darker skins have lots more natural melanin in them - the determinant of pigment - and are more resistant to burning in general. But be wary anyway. You could be burning even though you don't see any visible redness. Palms and the soles of feet are vulnerable to sun damage too.

Do I always use the same SPF?
No, you need to keep experimenting and using different sun protection factors to suit your skin. Have a few different levels of SPF protection on hand and adjust them to how your skin seems to be reacting now - not how it did a few years ago or even last summer. You change and so does your skin.

What kind of SPF do I use?
Always look for a sunscreen that is not water-soluble. Even moderate perspiration can wash away protection, not to mention swimming and water sports. However, the newest products are water-resistant or even waterproof to allow for long term, continuous water exposure without washing off which is great news for swimmers and people who don't want to reapply sun block or screen after every dip in the ocean or pool.