It's important to understand how sunscreen works in order to choose the most effective one. These days, there are so many letters on the bottle; it might as well be pre-calculus. Bur fear not! Here are some common terms and acronyms explained:

UVB: This stands for ultraviolet-B rays from the sun. UVB rays do not penetrate the skin too deeply, but are responsible for sunburns. Exposure to UVB rays is also responsible for basal and squamous cell carcinomas. These types of cancer are generally curable, but the treatment for them leaves scars on the skin.

UVA: Ultraviolet-A rays penetrate the skin much more deeply than UVBs do. While these rays don't cause any visible damage, they do result in long-term damage like wrinkling, leathering and other signs of photo aging. UVA rays also cause melanomas, a type of cancer which 62,000 people will be diagnosed with this year alone.

Sunscreen: Sunscreen absorbs UV rays. Using sunscreen is an effective way to protect yourself from UVB rays, but offers less protection from UVAs.

Sunblock: Sunblock deflects UV rays and is generally effective against both UVA and UVB.

SPF: Most people think that the SPF number shows how much protection a sunblock offers (it does stand for "Sun Protection Factor"), but it's really more about how long it can protect you. To be more exact, the SPF number measures the length of time the product protects you against UVB rays.

For example, sunblock with an SPF of 15 will protect you against the sun 15 times longer than if you were wearing no sunblock at all. So, if you would usually burn in 20 minutes, an SPF 15 sunblock will protect you for 300 minutes. That's why it's important to apply often.

Broad Spectrum: If you see this phrase on your bottle, it means that the sunblock blocks some UVA rays in addition to the normal UVB protection. However, it doesn't guarantee to block out all UVA rays.