Information is Beautiful has a superb post: The Sunscreen Smokescreen meant to answer the question “How much sunscreen should you wear?”
The full graphic can be found here. It begins by explaining UVA and UVB rays, goes on to explain SPF (UVA protection) and the star rating (UVB protection), and protection times avoided by sunscreen.
I cropped out the middle section which is specific to the amount of sunscreen which should be used and how often it should be reapplied. Not many (if any) of use use enough or reapply often enough.
The lower portion of the Information is Beautiful graph gives info on how cloud cover, reflective surfaces (snow, lakes, etc), and altitude affects the amount of UVA/UVB radiation. There is a section on skin cancers and one on the possible harmful effects of sunscreens.
Sun Protection (March 19, 2009)
Melanoma Review (February 25, 2008)
Melanoma Skin Screening Is Important (April 29, 2009)
Tanning Beds = High Cancer Risk (August 3, 2009)
Skin Cancer (March 24, 2010)
Safety of Sunscreens (June 14, 2010)
Dear 16-Year-Old Me (May 18, 2011)
New FDA Sunscreen Labeling (June 15, 2011)