SPF stands for Sun Protection Factor. The number you see actually represents the amount of time a sunscreen will protect you. An SPF 2 actually provides the same amount of protection as an SPF 40 does, the lower SPF will just protect you for less time.
Despite all of the confusion, it's relatively easy to determine the level of protection you'll get from a sunscreen. To find out how long a sunscreen will protect your skin, start with the amount of time it would take you to burn without sunscreen, then multiply (or Factor) that number by the SPF. This means that if you normally begin to turn red after 10 minutes in the sun, an SPF 15 would extend that amount of time by 15, which would be 150 minutes (or 2.5 hours). SPF 30 would give you 300 minutes (5 hours) of protection, and an SPF 90 would literally give more protection than there are hours of sunlight.
The caveat here is that sunscreen is by no means impervious to the elements. Experts recommend reapplying sunscreen every 2 hours since it will likely have worn off by then. Considering the short life of sunscreen on the body, and the fact that no one needs 16 hour sun protection, an SPF 90 seems pretty absurd- which is why most dermatologists will tell you that you need not bother with anything higher than a SPF 30.
There is one thing in support of using a product with a high SPF, however. Since most people do not apply enough sunscreen, using a higher SPF provides some cushion. A full ounce of sunscreen (about a shot glass full) should be used on the body- using less will provide less protection. If you choose a lower SPF you run the risk of not getting adequate protection. On the other hand, if you were to use even half of the recommended amount of SPF 70 you'd still get a sufficient block (about an SPF 35).
To get the best protection out of your sunscreen, choose one that has both UVB and UVA protection; these protect your skin from the sun's burning and aging rays. If you're concerned after hearing rumbles that several common sunscreen ingredients can actually cause free-radical damage, it's important to remember that sunscreen is still our best defense against skin cancer. If you're concerned, choose a formula with added antioxidants which will counteract free radical damage caused by the product. In my original post, I recommended Coppertone NutraShield. I've since realized that it stains clothing, so if you choose to try it, be aware! BTW, if you are using a separate treatment or moisturizer with antioxidants, these will provide the protection as well. Most importantly, use your sunscreen daily, and apply it liberally and often.