SPF stands for Sun Protection Factor and measures the protection against UVB provided by a sunscreen. An SPF 15, when applied properly, protects you against 93% of UVB. An SPF 30 protects you against 97% UVB.
SPF4 filters out 75% - 25% UVB gets through
SPF10 filters out 90% - 10% UVB gets through
SPF15 filters out 93% - 7% UVB gets through
SPF25 filters out 96% - 4% UVB gets through
SPF30 filters out 97% - 3% UVB gets through
SPF50 filters out 98% of UVB rays and SPF100 99%.
In reality there is actually not much difference between SPF15 and SPF50 – at least a lot less difference than the SPF figure appears to represent. The fact is that, as long as they are properly applied, most sun lotions offers quite a high level of protection and it just isn’t necessary for most of us to use an ultra-high SPF sunscreen.
SPF ratings higher than 30 are primarily used by sun care companies as a marketing tool, and take advantage of the public's misperception of what the protection ratings mean.