RJ stands for Registered Connector, which is a standardized physical network interface. Contains the construction of connectors and the wiring pattern for connecting data or telecommunications equipment to a service provided by a local exchange operator or a long distance operator. The standard designs for RJ45 connectors along with their wiring are called RJ11, RJ14, RJ21, RJ48, etc. Most of these interfaces are used around the world, but some are very common in North America.
RJ45 primarily uses the physical connectors comprised of the modular jack and the 50-pin miniature ribbon connector types. For example, RJ11 uses a 6-position, 4-conductor modular plug and jack, while RJ21 uses a 50-pin miniature ribbon connector.
RJ45 is considered to be the standard connector type used for network cables. These connectors are commonly found with Ethernet cables and networks. The construction of the RJ45 connector consists of 8 pins to which the wires of a cable are electrically connected. The standardized pinouts of this connector define the individual wire layout needed when connecting the connectors to the cable.
There are a few other types of connectors that look similar to RJ45 and can easily be confused when telling them apart. For example, the RJ11 connector used with telephone cables is slightly narrower than the RJ45 connector.
The RJ45 connector is commonly used in telephony applications and for network cabling. In special cases, it is also used for serial connections. Although this connector is used for a variety of purposes, it is primarily used for 10Base-T and 100Base-TX Ethernet connections.