Though plenty of people use sunblock and sunscreen interchangeably, the two terms don’t refer to the same products. The main differences have to do with what they contain and how they work.

The ingredients label on a tube of sunscreen no doubt lists at least a few chemicals, including—but not limited to—avobenzone, oxybenzone, ecamsule, octyl salicylate, homosalate, or octinoxate. As Health explains, these chemicals soak into your skin and create a screen (hence the name sunscreen) that absorbs the sun’s ultraviolet (UV) rays and prevents them from damaging your skin.

The main ingredients in sunblock, on the other hand, are zinc oxide and/or titanium dioxide, which both help block the sun’s rays from touching you at all. Instead, they reflect the light away from your skin. Because sunscreen protects you via chemical reaction, it’s sometimes called “chemical sunscreen.” And because sunblock acts as a physical barrier, it’s also known as “physical sunscreen.” In general, sunscreen is easier to apply, since your skin absorbs it; sunblock is thicker and more likely to leave a white sheen on your skin.

But knowing the difference between sunblock and sunscreen may not help you when you’re trying to choose the best product from the myriad options at the drugstore. One reason is that the FDA banned manufacturers from using the word sunblock on labels because it can give consumers an exaggerated idea of how effective a product actually is. And you might come across products that seem like a fusion of sunscreen and sunblock—for example, a certain lotion might contain oxybenzone and zinc oxide.

According to the American Academy of Dermatology, you should choose something that is water-resistant, has an SPF of 30 or above, and advertises “broad-spectrum protection.” Broad-spectrum means that it shields against both UVA rays—also known as “aging rays,” which can wrinkle and age your skin—and UVB rays, which cause sunburn.

[h/t Health]