If you want to see which process is using which port (in this case, port 80), you can do it with the following command:
# netstat -tulpn| grep :80
This gives the following output:
tcp 0 0 :::80 :::* LISTEN 8879/httpd
As you can see, apache (httpd) is using port 80 on PID 8879.
You can also do it with other ports by replacing the number (80) by another port.