There are three types of venomous snakes:
Opisthoglyph: These are the rear-fanged snakes, the fangs are enlarged rear teeth with a 'groove' that venom flows down while they are swallowing the prey item. They are mostly harmless or mildly venomous but there are two BIG exceptions. The Boomslang (Dispholidus typus) and the Twig snake (Thelotornis kirtlandi) have killed humans before. Other good examples of this type of snake are the Mangrove (B. dendrophila) and Hognose snakes (Heterodon ssp.)
Proteroglyphs: These are the fixed front fang snakes. These snakes have small non-movable front fangs. When they bite they hang on and 'chew' to envenomate the prey. Obvious examples of this type of snake are the cobras (Naja), kraits (Bungarus), mambas (Dendroaspis), and coral (Micrurus) snakes. These are some of the deadliest snakes in the world.
Solenoglyph: These snakes have movable front fangs. The fangs fold back into the mouth until they are needed. This is what makes these snakes more dangerous work with. They can grab on to your hand like a cobra would but they can also open their mouth almost 180 degrees with the fangs extended straight out. This enables them to strike at any portion of your body because it is more of a 'stab' than a bite. Examples include rattlesnakes (Crotalus), eyelash vipers (Bothriechis), gaboon vipers (Bitis), cottonmouths and copperheads (Agkistrodon)