A product's SPF or Sun Protection Factor represents the ability of its sunscreen to delay sun-induced burning. It is important to realise that an SPF is only a measure of how much UVB protection it provides. Unfortunately, a similar system of denoting sunscreen protection for UVA radiation has not been universally recognised, so always look for 'broad spectrum' sun products.
Sunscreens work by two mechanisms:
Chemical sunscreens: Absorb UV rays, lowering the energy level and releasing energy as heat. This type needs to be applied to the skin before any other product, and normally 20 minutes before sun exposure
Physical sunscreens: Reflect or scatter the energy rays. They are mineral based so therefore less irritating. They need to be applied last as they bounce the rays off the skin. You will also find physical sunscreens in mineral make-up, which is normally SPF 20-26, depending on the brand.