An RJ45 cable is the everyday name for a wired Ethernet network cable, usually twisted-pair copper (such as Cat5e, Cat6, or Cat6a), with a small clear plastic plug on each end that clicks into the network ports on computers, routers, switches, and wall jacks. The plug has eight positions and eight contacts, so it is technically called an 8P8C modular connector; it terminates the eight wires that make up the cable’s four twisted pairs.
The name comes from the telephone industry’s Registered Jack (RJ) standards. Strictly speaking, the original RJ45 was a slightly different telephone connector, but the very similar Ethernet 8P8C plug came to be called RJ45 across the industry. The plug shape stays the same across cable categories; how fast the connection can run depends on the cable category and the connected network adapters.
Source: Intel: Ethernet Cables and Transceivers Technology Guide
Image: Ethernet RJ45 connector by David Monniaux, CC BY-SA 3.0, via Wikimedia Commons.