Predominantly from warm, tropical reefs. A few live beyond the subtropics and some in brackish or even fresh water.
Pharyngeal jaws are in the pharynx, the throat. A few animals have a second pair of jaws in their throat, but Moray Eels (Muraenidae family) are the only animals that use theirs to grab hold of live prey and pull it down from the mouth and toward the oesophagus for swallowing.
It allows them to eat much bigger prey than they could otherwise manage. This is the kind of thing you have to do when you don't have arms, paws or hands, let alone a knife and fork!
Here's a slow motion video of a Moray Eel eating, and some more information on those jaws.
More on the Moray Eel.