SPF stands for Sun Protection Factor and is a rating for sunburn protection, mainly from UVB rays of the sun. However this rating is no help with the harmful UVA rays of the sun. UVA rays can cause skin damage and cancer.
For sunscreen to be effective, it must be applied properly. This means putting an ounce of sunscreen on 20 to 30 minutes before going out in the sun. Then it must be reapplied after two hours. Sunscreen must also be reapplied after going into the water or perspiring because it will wash off.
"The American Cancer Society recommends that people use a sunscreen with a SPF of at least 15, the American Academy of Dermatology opts for 30. Avoid sunscreens with SPF numbers higher than 50. More important than seeking out ultra-high SPF products is that you apply your sunscreen generously -- most people put on only a 1/4 to 2/3rds enough sunscreen to actually reach the product's SPF rating."
http://www.enviroblog.org/2010/06/understanding-sunscreen-3-questions-about-spf.html