RJ45 connectors have become ubiquitous in the modern world. The ‘RJ’ part stands for registered jack. A registered jack is a standardised interface used to connect the cabling in networks. There is a range of registered jacks, each with a slightly different design depending on the kind of cabling that is being connected, and the differences in these standards are delineated by the numbers after ‘RJ’; for example ‘RJ11’, or ‘RJ45’. To get a clearer picture of this range, try looking at the RJ45 connectors supplied by rs-online.com.
The original RJ45 connectors were used to connect to telephone networks. As computer technology developed, a standard way of connecting modems to telephone networks was required, and the original RJ45 connector was developed for this task. The arrangement of the pins in the jack made it possible to use standard telephone wiring to transmit data, both to and from the modem. In technical terms, the original telephone RJ45 connectors had an 8P2C array of pins, meaning that just two of the middle positions in the array were conductors.
As computer networking developed, an extremely similar standard physical interface was used for connection, the 8P8C. As you might guess, the 8P8C features conductors in all 8 middle positions. This difference is quite subtle to the naked eye, and thanks to the similarity between the original 8P2C RJ45, and the newer 8P8C RJ45, the name RJ45 became the standard way to describe the 8P2C connectors used in computing networks (like the Ethernet cable you use to connect your computer to your modem). For more information the history of rj45 connectors, try looking at: http://en.wikipedia.org/wiki/Rj45.