(Registered Jack-11) A telephone connector that holds up to six wires. RJ-11 is the common connector used to plug a telephone into the wall and also plug the handset into the telephone.

RJ-14 and RJ-16
RJ-11 jacks can hold six wires, but jacks and cables typically have four, which accommodates two-line phones (red-green and yellow-black wire pairs). When two lines are used, the jack is technically an RJ-14. When all six wires are used for three telephone lines, the jack is an RJ-25.

C's and W's
An RJ-14C refers to a desk phone jack, while an RJ-14W means wall mounted.