It is a port that has two data lines, RX and TX, and data is sent in a serial fashion across those data lines. It is used today for embedded systems, routers and switches et al, and getting a console on any machine that doesn't have a gpu with a monitor attached.
USB is a serial BUS, which allows multiple devices; serial ports are single device (if my memory serves).
USB is a serial BUS, which allows multiple devices; serial ports are single device (if my memory serves).