Sunscreens mostly protect against UV-B rays.
Sunblocks are physical blocks which absorb some rays and reflect back the rest.
The primary ones, titanium dioxide and zinc oxide, are now available in finely micronized form, which makes them transparent. Therefore, physical blocks, especially zinc oxide, are much more effective than chemical sunscreens in blocking UV-A rays and should be an important component of any sunblock. Skin cancer is going up, even while sunscreen use has been increasing. Some scientists believe one reason is the sunscreens stop UV-B rays from burning, so people stay in the sun longer, thinking they are protected. In addition, the UV-A rays harden the collagen fibers which support the skin.