All I can say right about now is OUCH.
SPF (the sun protection factor) lets you calculate how long you can stay in the sun before getting burned. (A fun, apparently well-known fact that I have missed my entire life up to this point. I feel about 3% smarter now). If, for example, it typically takes 10 minutes for you to burn, a sunblock with an SPF of 10 would allow you to stay in the sun 10 times longer (100 minutes) than if you didn't have sunscreen. A sunblock with an SPF of 45 would let you be in the sun 45 times longer (7.5 hours) before getting burned. The SPF factor refers only to the UVB (the shorter, sunburning) rays, doesn't account for skin type, and is not the standard for protection against UVA rays, which is in the works.
What started out as a simple "hey, world, now I know what the SPF factor actually indicates" post could easily expand into the difference between sunscreen and sunblock, or turn into an essay that explores the benefits of sun exposure for Vitamin D production in the body, a lecture on applying enough sunscreen (that lecture would be directed at me), a treatise on the UV Index (which the EPA makes easily available), or a summary of freaky skin cancer statistics.
But it won't. Instead, I'll conclude by mentioning that a recent New York Times article about sunscreen makers increasing the SPF factor caught my eye. Who needs an SPF of 100? Even if it does take as little as 10 minutes to burn, 1000 minutes (16.6 hours) is too long to go without reapplying.