A serial connector which looks very much like a standard telephone connector, except it houses eight wires instead of four.
RJ-45s are typically found on computers either integrated into the mother board or on a NIC. 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.
[Would the cable normally be shielded twisted pair or unshielded twisted pair?]