What do the different spf numbers in sunscreen and letters spf stand for?

SPF: Spf stands for sun protection factor. The number is determined experimentally indoors by exposing human subjects to a light spectrum meant to mimic noontime sun. Some subjects wear sunscreen and others do not. The amount of light that induces redness in sunscreen-protected skin, divided by the amount of light that induces redness in unprotected skin is the spf. It is mainly a measure of uvb protec.

Very misleading: Spf is a very misleading number, it only refers to the ability of a product to protect you from uvb rays. Those are the type of suns rays that can cause cancer. But spf does not in any way measure protection against uva rays. Those are the rays that cause burning and aging, and can lead to cancer as well. You need a sunscreen of no more than spf 50 with either zinc or titanium in it. Reapply often.