Source : Free On-Line Dictionary of Computing

RJ-45
     
         A {serial} connector which looks very much like a
        standard telephone connector, except it houses eight wires
        instead of four.  RJ-45s are not typically found on
        {computers} or normal serial equipment, but because they are
        so small they are often used on devices such as {terminal
        servers} that have many {ports}.
     
        {Ethernet} ({10baseT}) and {Token Ring} sometimes use four
        wires of an RJ-45 plug, {100baseVG} uses all eight.
        {100BaseTX} uses the same four wires of the RJ-45 connector as
        10baseT but the wire must be {category 5} instead of {category
        3}.
     
        (1998-06-30)