SPF (Sun Protection Factor)- the number of times it takes for the skin to redden. (NOTE: if a sunscreen does not say "broad Spectrum", it means it will only protect against UVB rays.) ex: SPF 15 (protection from 93% of UVB rays
SPF 30 (protection from 97% of UVB rays
SPF 50 (protection from 98% of UVB rays
... and so on...