A TCP/IP connection is always made to an IP address (you can think of an IP-address as the address of a certain computer, even if that is not always the case) and a specific (logical, not physical) port on that address.
Usually one port is coupled to a specific process or “service” on the target computer. Some port numbers are standardized, like 80 for http, 25 for smtp and so on. Because of that standardization you usually don’t need to put port numbers into your web adresses.
So if you say something like http://www.stackoverflow.com, the part “stackoverflow.com” resolves to an IP address (in my case 126.96.36.199) and because my browser knows the standard it tries to connect to port 80 on that address. Thus this is the same as http://www.stackoverflow.com:80.
But there is nothing that stops a process to listen for http requests on another port, like 12434, 4711 or 8080. Usually (as in your case) this is used for debugging purposes to not intermingle with another process (like IIS) already listening to port 80 on the same machine.