RJ45 is the common name for an 8P8C modular connector using 8 conductors that was also used for both RJ48 and RJ61 registered jacks (which specify pin assignments of the wires in a telephone cable), although "RJ45" was not originally specified as a registered jack with today's Ethernet wiring. The "RJ45" physical connector is standardised as the IEC 60603-7 8P8C modular connector with different "categories" of performance, with all eight conductors present but 8P8C is commonly known as RJ45. The physical dimensions of the male and female connectors are specified in ANSI/TIA-1096-A and ISO-8877 standards and normally wired to the T568A and T568B pinouts specified in the TIA/EIA-568 standard to be compatible with both telephone and Ethernet.
A similar standard jack once used for modem/data connections, the RJ45S, used a "keyed" variety of the 8P8C body with an extra tab that prevents it mating with other connectors; the visual difference compared to the more common 8P8C is subtle, but it is a different connector. The original RJ45S keyed 8P2C modular connector had pins 5 and 4 wired for tip and ring of a single telephone line and pins 7 and 8 shorting a programming resistor, but is obsolete today.
Electronics catalogs commonly advertise 8P8C modular connectors as "RJ45". An installer can wire the jack to any pin-out or use it as part of a generic structured cabling system such as ISO/IEC 15018 or ISO/IEC 11801 using RJ45 patch panels for both phone and data. Virtually all electronic equipment which uses an 8P8C connector (or possibly any 8P connector at all) will document it as an "RJ45" connector.
Read more about this topic: Registered Jack