RJ45 is a physical interface often used for terminating twisted pair type cables. 'RJ' stands for Registered Jack which is part of the US 'Code of Federal Regulations'. It has eight 'pins' or electrical connections.
It is usually used with standards like TIA-568B which define the wiring pinout.
A very common application is its use in Ethernet patch cables where usually 4 pins (2 pairs) are used. Other applications included termination of business telephones and other networking services such as ISDN and T1ss.