Here is some documentation that will explain technical details.
Since a directory is a list of files and subdirectories, an index page is meant to show this list when browsing to a directory. It doesn't have to though and you need to follow the link to learn how.
By default the web server will redirect a URL like http://www.domain.com/newpage to http://www.domain.com/newpage/ assuming you have a directory at /home/user/domain.com/newpage/
So you could just link to http://www.domain.com/newpage/ and create an HTML document at /home/user/domain.com/newpage/index.html and be done with it.
The trailing slash is necessary as you are linking to a directory ("folder") instead of a file.