The phosphorus atom in phosphorus trichloride possesses a lone pair of electrons, which causes the atom to be polar. The three chlorine atoms bonded to the phosphorus atom exert a pull on the molecule's electrons, while the lone pair does not.
Phosphorus trichloride has a pyramidal structure, with the phosphorus atom in the center. Chlorine is very electronegative and pulls electrons closer to it. The lone pair of electrons, however, doesn't pull electrons toward itself; in fact, it repels them. The net result is a movement of electron density away from the lone pair, which creates a center of positive charge. Conversely, the area between the three chlorine atoms becomes a center of negative charge, making the atom polar.