<p>In Linux, what is the difference between <code>/dev/ttyS0</code> and <code>/dev/ttys0</code>?</p>

<p>I know that the first is a serial port, but what about the second, with the small <code>s</code>?</p>

see this
For a pseudo terminal pair such as
  ptyp3 and ttyp3, the pty... is the
  master or controlling terminal and the
  tty... is the slave. There are only 16
  ttyp's: ttyp0-ttypf (f is a
  hexadecimal digit).
To get more
  pairs, the 3 letters q, r, s may be
  used instead of p
. For example the
  pair ttys8, ptys8 is a pseudo terminal
  pair. The master and slave are really
  the same "port" but the slave is used
  by the application program and the
  master is used by a network program
  (or the like) which supplies (and
  gets) data to/from the slave port.