Sunscreen is a topical (not to be confused with tropical, though sunscreen is very useful in the tropics) solution (usually available in a spray, cream, or lotion) which protects the skin from ultraviolet solar radiation. Each brand of sunscreen has a different composition, but the most commonly found ingredients are zinc oxide, titanium dioxide, oxybenzone, dioxybenzone, and other chemicals with "benzo-" in them1. The effectiveness of a particular type of sunscreen is called it's Sun Protection Factor or SPF, a number which ranges between 2 and 60. This number is calculated using human testing, where they find out how long it takes a person's skin to redden both with and without the sunscreen. The SPF is the ratio of the time to burn with sunscreen (in minutes) divided by the time to burn without sunscreen, so if I burn in 5 minutes of sunlight without sunscreen, and in 45 minutes with sunscreen, then the sunscreen has an SPF of 9. It should be noted that the SPF does not rate the amount of protection, but rather the duration of protection - a sunscreen with SPF of 60 will not block 6 times as much UV radiation as an SPF 10 sunscreen, but should last about six times as long. Since UV radiation can cause painful, disfiguring, or even fatal ailments such as skin cancer, cataracts, or macular degeneration you should wear sunscreen whenever you are in the sun.
N.B. - A popular essay entitled "Advice, like youth, probably just wasted on the young" by Mary Schmich describes wearing sunscreen as the one piece of advice she would like to give as part of a commencement speech. The essay gained notoriety when Baz Luhrmann set it to music, calling it "Everybody's Free (To Wear Sunscreen)".
1. denoting, no doubt, their descendence from benzene, a clear, flammable, poisonous, aromatic liquid obtained by scrubbing coal gas with oil and by the fractional distillation of coal tar.