Correctly wired, the box will contain two (out of phase) hots and one neutral. The receptacle is constructed so that one side of the receptacle has two connected wire-mounting screws (as in a duplex), while the other side has two separated screws- one for the other hot, and one for the neutral. It is powered from a two-pole breaker; I understand current code to require either a handle tie, or an internal connection between the two trip mechanisms.
A common, inncorrect, unsafe method is to simply ground the neutral screw- usually with a piece of wire from the neutral screw to the ground screw. This method has all the neutral current flow through the mounting screws.
