By now, I hope you all know the SPF stands for sun protection factor. A few sunscreen labels also list a "PA" ranking, like this Shiseido sunscreen pictured. PA ranking refers to the amount of protection the sunscreen offers from the UVA rays, since SPF just ranks protection from UVB rays.
PA rankings are listed as PA+, PA++ or PA+++ with the more plus symbols the more protection from UVA rays, the rays that may cause long term skin damage and therefore age the skin.
Always look for a broad spectrum sunscreen as they will protect you from both UVA and UVB rays.
You can find the Shiseido sunscreen, rated SPF 55 P+++, for $37 from Sephora.