Source : Free On-Line Dictionary of Computing

first-in first-out
     
         (FIFO, or "queue") A data structure or hardware
        buffer from which items are taken out in the same order they
        were put in.  Also known as a "shelf" from the analogy with
        pushing items onto one end of a shelf so that they fall off
        the other.  A FIFO is useful for buffering a stream of data
        between a sender and receiver which are not synchronised -
        i.e. not sending and receiving at exactly the same rate.
        Obviously if the rates differ by too much in one direction for
        too long then the FIFO will become either full ({block}ing the
        sender) or empty ({block}ing the receiver).  A {Unix} {pipe}
        is a common example of a FIFO.
     
        A FIFO might be (but isn't ever?) called a LILO - last-in
        last-out.  The opposite of a FIFO is a LIFO (last-in
        first-out) or "{stack}".
     
        (1999-12-06)