SPF stands for Sun Protection Factor and the protection offered by sunscreens differs depending on the SPF you use.
What percentage of UV rays are blocked by SPF?
SPF 15 = 93%
SPF 30 = 97%
Remember a higher SPF does not mean you can spend longer in the sun.
Always apply sunscreen 15 minutes before going into the sun or water. Reapply every two hours, or every 40 minutes if in the water or sweating.