Sunscreens and sunblocks are formulated to protect our skin from the damaging ultraviolet radiation of the sun. Even on cloudy days UVA rays can age our skin and UVB rays can burn our skin.
Sunscreens are chemical filters. These clear lotions bond with the skin and absorb some of the UVB (and sometimes UVA) rays before they can penetrate the surface of the skin. Think of them as a screen door on a house, stopping some things and allowing others to pass through.
A sunblock is an actual physical barrier that you spread on top of your skin. These opaque creams typically contain metallic compounds such as zinc oxide or titanium oxide which completely reflect both UVA and UVB rays. If sunscreen is a screen door then sunblock is a solid oak door, totally separating your skin from the sun.
Pros and Cons
Sunscreens provide less overall protection, but are cosmetically less obtrusive. They can even be worn under make-up. Because they are clear, however, it can be difficult to know when your skin is adequately covered.
Sun Protection Factor (SPF)
All sunscreens are rated according to their ability to protect you from UVB rays. A sunscreen with an SPF of 10 would allow you to spend 10 times as long as normal in the sun without burning.