RJ stands for registered jack which is a standardised networking interface.
RJ45 connectors are used for connecting Ethernet cables to equipment in computer networking applications such as telecommunications or data equipment. RJ45 connectors consist of modular jacks and plugs. The RJ45 plugs are connected to each end of an Ethernet cable. These plugs are then inserted into the RJ45 jacks on the Ethernet device.
Several other kinds of connectors closely resemble RJ45 and they can be easily confused for one another. The RJ11 connectors used with telephone cables, for example, use six position connectors rather than eight position connectors, making them slightly narrower than RJ45 connectors. RJ45 connectors have an 8P8C (8 position, 8 contact) configuration. In order for the connector to function properly the individual Ethernet cable wires must be inserted into these locations following a T-568A or a T568B wiring standard.
Technically, 8P8C can be used with other types of connections besides Ethernet; it is also used with RS-232 serial cables, for example. However, because RJ45 is by far the predominant usage of 8P8C, industry professionals often use the two terms interchangeably. RJ45 connectors are available to suit a range of LAN categories from Cat3 to Cat6a. Different mounting options are also available, including cable mount, PCB mount and panel mount.