Sunscreens protect your skin by absorbing and/or reflecting UVA and UVB rays. The U.S. Food and Drug Administration requires that all sunscreens contain a Sun Protection Factor (SPF) label. The SPF reveals the relative amount of sunburn protection that a sunscreen can provide an average user (tested on different skin types) when correctly used.
Sunscreens with an SPF of at least 15 are recommended. You should be aware that an SPF of 30 is not twice as protective as an SPF of 15; rather, when properly used, an SPF of 15 protects the skin from 93% of UVB radiation, and an SPF 30 sunscreen provides 97% protection.
Although the SPF ratings found on sunscreen packages apply mainly to UVB rays, many sunscreen manufacturers include ingredients that protect the skin from some UVA rays as well. These “broad-spectrum” sunscreens are highly recommended. UVA rays penetrate deep into the skin and heavily contribute to premature aging. UVB rays mostly affect the surface of the skin and are the primary cause of sunburn.
— Source: U.S. Environmental Protection Agency