Web server can run on any port. But the default port is 80. When you type in the URL of a web site the browser uses the default port 80 and connects to it. There are lot of occasions when other ports are used, mostly when more than one web server is needed in the same machine. Like webmin uses port 10000 by default.
For any web server not using port 80 you have to specify the port also with your url like http://yoursite.com:port
in case of default webmin install it is like http://localhost:10000
Note:HTTP uses 80 and HTTPS uses 443 by default.