It is cumulative damage to the skin. Eighty percent of exposure to the sun is incidental - from walking to the car or standing outside. Skin cancer is the most prevalent type of cancer, but is the most preventable. First, know when the sun's rays are the strongest - between 10 a.m. and 3 p.m. When you do go out into the sun, cover up and wear a wide-brimmed hat.
Most cancers occur on areas that are exposed to the sun.
Next, use sunblock - don't forget your ears and the back of your neck. Most importantly, check your skin monthly. Also, know how to use sunscreen. Anything more than 30 only provides about two percent extra protection, yet costs much more. Don't use tanning beds.