A registered jack (RJ) is a standardized network interface for connecting voice and data telecommunications equipment. Although the specification includes hysical construction, wiring, and signal semantics, the terms are often used loosely to refer to the modular connectors. So this comparison focuses on the modular connectors used in RJ11 and RJ45 jacks.
RJ11 is the most widely-used registered jack in telecommunications. RJ11 has a 6P2C or 6P4C configuration, and is used for wiring a single phone line. RJ45 uses an 8P8C configuration. In fact, the RJ45(S) jack — as defined in the standardized spec — is rarely used, but the RJ45 commonly refers to any 8P8C modular connector used in computer networking (Ethernet). It is this meaning of RJ45, the modular connector, that is being compared here. The specification that RJ45 in ethernet adheres to is called the TIA/EIA-568.
