Snorkels are used to breath while your face is in the water and are available in many styles, types, and sizes. Take some time to understand the differences, so that you find one (or two) that fit the shape of your mouth and have the features you desire.
Snorkels are either one solid tube, or a solid and a flexible silicon tube, along with a mouthpiece. They can have a splash guard or dry valve, a purge valve, and a mask strap clip. Let's take a look at the details:
A solid tube is rigid and fixed in a specific position. The cheap ones you'll find at rental places can be a simple shape that is very uncomfortable, or, maybe it has a bend to making it a little easier to use. A solid tube attached to a flexible silicon tube, however, can be adjusted (bent) and rotated to fit, offering more comfort to your mouth.
Breathing in a mouth full of water is not a pleasant experience. Both splash guards and splash valves sit on top of the snorkel tube and are used to keep that water out. Splash guards help keep water spray and splashes out of the tube, but will not stop water from entering when you dive or are covered by waves. Splash valves are good for pools and calm waters. Dry valves do a much better job of sealing out water and work well for waves and deeper waters.
Purge valves do just what their name implies. They purge water out of the tube when you blow! Without a purge valve, you have to blow water that gets inside, all the way up the tube. Purge valves are at the bottom, help you save your breath, and make it easier to expel the water. Remember to clean your purge valve before and after each snorkel adventure, to keep it working well.
Silicon is flexible, will last much longer than hard plastic, and will be easier on your mouth. Some can be pulled off and replaced if they get chewed, or for hygiene purposes i.e. if you are sharing snorkels with others. Look for a mouthpiece that fits your mouth properly, as they are available in sizes from adults, to teens, to kids.
Mask straps keep snorkels attached so they don't get lost in the water. For travel, find the quick release type, to make packing easier.