nc [hostname] [port] is like telnet: it goes there. But error messages go to stderr, so scripting is easier.
nc -l [port] listens. You can still pump data into stdin, and it will go to whoever connects.
This is handy to see what your browser/application is sending.
-q 0 means quit zero seconds after reading EOF on stdin. That is useful with -l if you are just piping something hardcoded.
A complete example:
nc -q 0 -l 8011 <<EOF
HTTP/1.1 200 OK
Content-Type: text/plain
hello world!
EOF