I've pasted an explanation of physical vs. chemical sunscreens below. Many people who have sensitive skin or want to stay away from synthetic chemicals opt for physical sunscreens.

Physical filters include zinc oxide and titanium dioxide. They work by sitting on top of skin and either reflecting or scattering UV rays. Zinc oxide can block against both UVA and UVB rays, covering the whole spectrum, while titanium dioxide only delivers protection from UVB rays. Both ingredients work well, but have a tendency to leave a white cast on skin. Newer, micronized versions or those that contain nanoparticles appear much more clear on skin.

Chemical filters include a range of ingredients including avobenzone (also known as Parsol 1789), oxybenzone, Tinosorb M, Tinosorb S, Mexoryl SX and Mexoryl XL amongst others. These agents work by interacting with the skin’s top layer to absorb UV rays and convert them into energy before they can harm skin. Chemical sunscreens tend to use a combination of ingredients to protect against the full spectrum of UV rays. While older chemical filters tended to protect well against UVB rays, they often didn’t deliver much in the way of UVA protection. Newer filters such as Mexoryl and Tinosorb now deliver UVA protection so look for sunscreens containing those. Avobenzone also offers UVA blocking capability, but one problem is that it tends to degrade upon exposure to light. If you are using a sunscreen that contains avobenzone ensure that it is stabilized with an ingredient such as Octocrylene as found in Anthelios sunscreens.