These are commonly known as sunscreens. The first and most common of the absorption chemicals is @PABA (para-aminobenzoic acid). It absorbs UV-B, one form of ultraviolet light.
All sunscreens are labeled with an @SPF, or Sun Protection Factor. This applies to cream's UV-B blocking ability. The SPF acts like a multiplying factor. If you would normally be OK in the sun for 10 minutes and you apply an SPF 10 sunscreen, you will be OK in the sun for about 100 minutes. In order for the sunscreen to work, however, you have to apply plenty, and it has to stay on. You should apply it about half an hour before going out in the sun (or the water) so it can bind to your skin -- if you don't, then it is very easy for the sunscreen to wash off.
Regardless of the SPF sunscreen you use, however, and regardless of the degree of your tan, moderation is the key. Even with these protective factors, sunlight can still cause a variety of problems from skin drying to skin cancer. Use your judgment with how long you stay out in the sun before taking a break.
