What it's for!
Allows you to breathe, while at the surface, without lifting your head up, so you do not have to take your eyes off the game. In addition it acts as a mouthguard, protecting your mouth. It Must be made of a pliable material.

Things to Check
Bore
The snorkel must have a reasonably wide bore (at least 2cm internal diameter). This makes it easier to breathe through and clear. Check this by seeing if your little finger can be easily inserted right into the snorkel through the top end, not the mouthpiece. The mouthpiece hole is often a different shape, check that this is not excessively small also. Look at a really expensive one with fancy valves on and check that the bore is no less.

Flexibility
The snorkel must be flexible, not made of hard or thin scratchy plastic. It is quite hard these days to get hold of a 'legal' flexible snorkel, they are rarely cheap.

Mouthpiece teeth-grips
Avoid mouthpieces with little tooth-grips, they get bitten off easily and are often positioned for the front (incisors) to hang on to. This provides a relatively weak grip and leaves your teeth feeling like they are sticking out at funny angles after a long game. Most decent snorkels have the grips positioned further back so that the premolars and molars can grip them, go for this sort.

Rubber or silicone?
It doesn't really matter what the main tube (or 'barrel') is made of so long as it is flexible. The mouthpiece however is a matter of preference. Black rubber tends to taste like... black rubber, and can leave your mouth feeling like a twenty wheeler truck has been doing donuts on your tongue, but is otherwise OK. Other mouthpieces are usually all right but as with masks the rubber tends to accumulate cultures of green-brown slimes which are easily cleaned off the silicone mouthpieces.

Head-hugging curves
Some snorkels have a straight tube, others are curved or 'contoured'. The curve is to keep the snorkel more flush with your head, this is theoretically useful in hockey as it reduces the chances of the snorkel being caught on other players and pulled off, but I doubt that it really matters. It does look a little nicer than the straight tube though.

The mouthpiece tube should be able to turn in at least one place so that its angle relative to the main tube can be adjusted. Check that this is not too loose, you don't want the tube and mouthpiece getting separated during a game, a lungfull of water is not nice. It is also advisable to make sure that the mouthpiece tube is flush to the face and made of a relatively firm material so that it does not bend when you swim. Many cheap snorkels have a big U bend in them, avoid these, they drag and bend so you cant breathe while swimming at any speed on the surface and snag on anything that comes near you in a game.

Length
This doesn't matter when you first buy a snorkel, because it will undoubtedly be too long. For hockey the snorkel must be cut down so that it does not protrude far above the head. This is so that you do not injure other players with an excessively long snorkel, but it has many other advantages too. Drag is reduced, swimming underwater with a snorkel that is too long tends to cause annoying vibrations as the snorkel tries to lever your jaw out. Plus it reduces the chances of your snorkel getting knocked or caught by another person, resulting in your other headgear, mask and hat, being torn off.

Purge & backwash valves
Some snorkels have valves near the mouthpiece. The idea is that when you blow the water out of your snorkel it blows out of the top and the bottom, making it easier to clear. Some snorkels have other valve gadgets up the main tube to catch any water that flows down and guide it out rather than into the mouthpiece.

Do NOT buy a snorkel with any kind of valves or gadgets up the main tube. The snorkel must be cut down to an appropriate length anyway so they will be cut off. So don't go out and get a fancy $90 U.S. Divers Impulse with the backwash-catch-cone on the top, you'll have to cut it off to meet the length requirements, and the tube is too hard anyway so it's totally against the rules.

Snorkels with a purge valve just below the mouthpiece are OK but unnecessary and they often cause drag which is uncomfortable and can cause the mouthpiece tube to fold so you can't breathe when moving with any speed on the surface. The advantage gained by having a purge is minimal, so, as with mask purge valves, avoid them.

Snorkel keepers
These are little loops of rubber that you use to attach your snorkel to your mask strap or hat ear-guard. They have a tendency to break now and then so carry a spare, or at least a thick, short elastic band. Make sure that the keeper holds the snorkel tightly to your mask or hat so that it does not allow it to wobble around too much