It takes only one long day at the beach to realize the importance of sunscreen. Sunscreen protects against two types of rays: UVA and UVB. The latter causes sunburns, but any product with an SPF above 15 should protect you. Anything higher gives you marginally better protection against burns but does a better job of reflecting the UVA rays that are associated with skin cancer.
But don't breathe a sigh of relief just yet. found many sunblocks have a significantly lower SPF than advertised. We took the worst offenders (products whose actual SPF was less than half of what they claimed) and made this handy graphic, so you know which brands to steer clear of the next time you're shopping for sunblock.