What do you mean by UVA and UVB?
 

Filters in sun screens block two kinds of sun rays that we know to be instrumental in the development of skin cancer: UVA and UVB.

UV is the abbreviation of ultraviolet and radiation comes in two different versions referred to as A and B.

A handy little mnemonic:

UVA = aging (skin)
UVB = burned (sunburn)

The sun protection factor (SPF) number indicates the percentage of UVB rays that the filter blocks. UVA protection will always be at least 1/3 of UVB protection, so a sun screen with SPF 15 protects against UVA radiation with at least SPF 5.