If the port is open on your computer and you want to see what is using it, you can use
netstat -b
from the command line.
See
Netstat[
^] options on Wikipedia.
If the service is not running and you want to know what might use that port, then see the
List of ports[
^] on Wikipedia
If you want to find what protocol is running on a port on a remote computer, use something like
nmap[
^]