The old fashioned fixed snorkels are more or less gone. Most snorkels today incorporate a flexi-hose between the mouth piece and the breathing tube. This allows the hose to be bent out of the way when diving and using the regulator. The more expensive snorkels have a smooth inside the flexi hose, this is to minimise turbulence and breathing resistance inside the tube.
Modern snorkels have a purge valve (a butterfly valve) at the bottom of the snorkel near the mouth piece. When clearing the snorkel, the water gets pushed out of the valve and doesn't need to be transported up the tube and "blown" out. The most effective way is to squeeze the flexi-tube shut when purging your snorkel.
The tube should not be too long. You have to remember that the snorkel holds exhaust air, which means the initial part of your breath is "used" air, with a higher level of carbon-dioxide (CO2). So longer the tube, so harder you will need to breath to get "fresh" air. The tube should be approximately 4 to 6 inches (10 to 15 cm) above your head when you are face down in the water.
Wave Chamber and Tube valve
Very cheap snorkels, especially those from about 20 years ago, had a ping-pong ball stuck at the top of the tube shutting the tube-end when going underwater - modern snorkels have well designed valve at the tube end. However, this can cause a feeling of suffocation when all of the sudden you can't breath. Also, most people will hold their breath when water enters their mouth. Most modern snorkels will have a small wave chamber near the mouth piece, where a small amount of water can accumulate and purged through the purge valve.
Our favourite snorkel
Most divers don't like having a snorkel, it seems to get in the way and we have even lost snorkels when diving. A fantastic alternative is the Oceanic Pocket Snorkel. It can be put easily in your BCD or suit pocket and be pulled out when needed. Also, you will find the more expensive snorkels tend to be made of different parts (allowing to some extend replacement of broken parts) - this is quite important if you wish to change the mouth piece of your snorkel (e.g. to get a comfort bite fitted).