SPF means Sun Protection Factor.  It is the number on your sunscreen that tells how well the sunscreen protects you from UVB radiation.

What coverage does SPF offer?  Is it helpful to get a really high number SPF?

No sunscreen offers 100% protection.  In the lab, SPF 100 blocks 99% of the UVB rays.  SPF 50 blocks 98% of UVB rays and SPF 30 blocks 96.7% of the UVB rays.  So increasing the number only gets slightly more protection, if applied as directed.

How is SPF calculated?

SPF is calculated by comparing the time needed for a person to burn unprotected in the sun, with how long it takes for that same person to burn while wearing the sunscreen.  If you normally burn after 10 minutes in the sun without sunscreen on, then with SPF 30 you should be able to stay in the sun for 300 minutes.

How much sunscreen is enough?

Wearing enough sunscreen is important.  The American Academy of Dermatology recommends one ounce (a shot glass full) of sunscreen to the body, with repplication every 120 minutes if swimming, or getting wet, or sweaty.  If you apply half the amount of a given SPF sunscreen, you get the protection of only the square root of the SPF.  If you are using an SPF 70 and you apply 1/2 ounce, it will give you an SPF of 8.4! 

Other facts -

SPF in different products is not additive.  If you have make up with SPF 15 and sunscreen with SPF 20, your level of protection is only 20 times unprotected.  They do not add to SPF 35. 

Sunscreen is important on cloudy days since UVB and UVA rays penetrate clouds. 

SPF does not tell anything about how a sunscreen absorbs UVA rays.  These rays of the sun penetrate deeper into the skin and are associated with skin cancer formation.  By blocking the UVB and preventing burning, some people stay outside longer and get more UVA exposure, thus fueling the myth that sunscreen causes skin cancer.  It is in fact the UVA rays that are causing the problem.