SPF, or Sun Protection Factor, measures how well a sunscreen protects against UVB rays, the type of UV radiation that causes sunburn. A higher SPF offers more protection, but no sunscreen can block 100% of UV rays. Broad-spectrum sunscreens are recommended because they protect against both UVA and UVB rays. UVA rays penetrate deeper into the skin and contribute to aging and skin cancer. Look for a sunscreen labeled 'broad-spectrum' with an SPF of at least 30 for effective protection.