In short the RJ45 is the bit of plastic that goes on the end of a telecommunications or computer networking cable so that it can be connected to an application. It is the bit of plastic that plugs your telephone cable into your telephone.

The acronym RJ refers to registered jack and applies to a large number of types of plastic connector. These can be a variety of different shapes and sizes depending on the cable type and the port into which they need to fit.

The RJ45 generally refers to the connector that is used in the standard network Ethernet connection. The Ethernet is a network of communication cables and it is how computers talk to each other. There are to significantly different types of cable that feed into the RJ45 connector. The first is a Cat 4 connection that enables four wire connections and the Cat 6 that enables six wire connections.

In the RJ45 cat 6 the cable contains 6 wires that can be used for data transfer. Out of these six internal wires only four are actually used. The manufacturers have added an extra two in anticipation of improvements in the technology industry. They have predicted that in the future the cables will be required to carry greater amounts of data but at the present moment they are not all used. This is similar to the theory of redundancy the RJ45 doesn't use the extra wires until they really need to.

The registered jack connections are used in standard network cards and computers that connect to networks. The RJ45 cable is an industry standard as opposed to the RJ11 that only utilises 2 of four wires in a phone.

Registered Jacks were first introduced in the 70s and replaced the bulkier connections that were previously used. The American Telephone and Telegraph Company introduced the plug under the Bell system which was a trade mark they used to co-brand the services they offered.

The RJ45 provides a much cleaner and aesthetically pleasing connection in comparison to the old style phone connectors that were very bulky. RJ connectors that are suffixed by the letter C are flush mount or surface mount the letter W is wall-mount and the S means single jack. M is a multi-line and X is a complex jack.