the dose of UV radiation required to produce 1 minimal erythema dose (MED) on protected skin after the application of 2 mg/cm2 of produce divided by the UV radiation to produce 1 MED on unprotected skin.
In other words, it's how much of this stuff you need to use to protect your skin from a dose of radiation that would affect unprotected skin. The higher the number, the more protection you should get.
"Water resistant" sun screen must maintain its SPF after immersion in water for 40 minutes.
"Very water resistant" sunscreen must maintain its SPF after immersion in water for 80 minutes.
Broad spectrum or full coverage sunscreen will protect against both UVB and UVA rays.
So why do we care about sunscreen? Because it is one of the key things you can do every single day to slow down the photo-aging process (join me tomorrow for that topic...)