1. First, a quick science lesson because this stuff is confusing.
Sunlight is composed of two types of rays, ultraviolet A (UVA) and ultraviolet B (UVB). While both play key roles in causing skin cancer, most people are only concerned with UVB rays on a day-to-day basis, which burn the superficial layer of the skin and cause sunburns. UVA rays are more damaging in the long run, as they penetrate far deeper into the skin and can cause skin aging and wrinkling.
I'm sure we've all heard of SPF, which stands for "Sun Protection Factor." This refers to a sunscreen's ability to shield you from UVB rays, and it's based on how much longer is would take to get a burn while wearing sunscreen vs. not wearing sunscreen. Because SPF only blocks UVB rays, it's important to buy "broad spectrum" sunscreens, which block out both UVB and UVA rays.