There is a limited amount of space available on a web page. It may not seem that way, as you can scroll down. However, it's one thing to scroll down when scanning images on a web site for a product that you want, and another when reading content. Most visitors will not sit and read an endless page; they'll 'bounce'. This is the term used to describe the act of navigating away from your site, and that is something that you don't want to happen.
A solution is to create a small article, a Story, that contains a reference to, or subset of, the original piece in the form of a Read More link. A Teaser is when you show enough Node Content to create an interest in the remainder of the Story. Drupal allows you to determine exactly how much Node Content you want to show. Essentially, you are putting a 'mark' in the Page or Story. This way, you can have a five-paragraph Story, but only display one paragraph on the front page. There aren't two versions or copies of the piece, just...