Most Serpentes have teeth, but there are some that have a more specialized form of teeth that they use to inject venom called fangs. These fangs come in 3 different forms, Solenglyphous, Proteerglygphus and Opisthoglygphus.
Solenglyphous fangs are a type of fang that folds into the Serpentes mouth when closed. This allows the fangs to be very long and give them larger reach without getting in the way of there mouth.
Proterglyphus fangs are fixed fangs in the front of the mouth. Think of these as two sharp syringes that when punctured into a foe cause them to become envenomed.
the last kind is Opisthoglyphus or more commonly known as rear fangs. These fangs are in the back of the snakes mouth and are used more in helping to eat then in defense. However, they can still give a nasty bite when scared so watch out.