There are two main types:
A snorkel that attaches to your mask. (The best place for it).
A folding snorkel that fits in your BC pocket for those that are irritated by a snorkel hanging at the side of their face.
This counts, you get little bores for kids and bigger ones for adults.
Can be made of cheaper Silita which is hard, or soft silicone.
Has a flexible tube or part that makes it fall away from the mouth when not being used.
A one way valve that allows water to drain out, making it easier to clear.
A fitting at the top that diverts water and spray from the breathing tube.