RJ45 is used with Ethernet 100BaseT networks.
The standard connector for unshielded twisted pair cabling is an RJ-45 connector. This is a plastic connector that looks like a large telephone-style connector (Ethernet type).
A slot allows the RJ-45 to be inserted only one way. RJ stands for Registered Jack, implying that the connector follows a standard borrowed from the telephone industry.
This standard designates which wire goes with each pin inside the connector.