You have a DSL line and have configured your router to only allow requests directed at port 80, served by your public web server. You can choose any port number as long as the port is not reserved for use by another service, and the number is greater than 1023 if you're not running as root. To visit a web site hosted on a port other than 80, your visitors must include the port number in the URL they type into their browsers. Be sure to set the ServerAdmin directive so that your visitors don't have to resort to guesswork to report problems they have with your site. This is where you would store the files you wish to make available for public viewing.
Most folks use the directory as the root of an organized hierarchy of directories.
For more information on some of the dynamic content generators I mention here, visit the Resources section at the end of this article. The ScriptAlias directive indicates that anything in the specified directory should be run as a program rather than simply sent to the browser as a file. Behind the scenes, the web server is looking in the document root directory for the file specified in the DirectoryIndex directive to display by default. If you're used to the Windows three-character file suffix limit or are in an environment where some folks will be editing documents for online publication under Windows and uploading these to your server, you might use index.htm as your default. Some servers assume a default.html or default.htm document.