UVB causes sunburn. UVA does other skin damage. SPF 15 means your skin is protected from sunburn for up to 15 times longer than if you weren't wearing sunscreen. Lower SPF numbers than 15 mean that a sunscreen will help your skin tolerate the sun for shorter times. Higher SPF numbers protect your skin for longer times.
Different products can have different active ingredients.
This tells you special product information. A person may be allergic to an ingredient as sometimes happens with the ingredient PABA, which can cause skin rashes.