A Teaser is usually posted/promoted, on the front page of your web site such that the first 600 words are displayed with a link to the full article.

You can change the number of words for the teaser under Administer > Settings > Posts. Although, once you do this, you must open and resubmit old articles to set the new teaser length.

Another, and more convenient, way to set the teaser length is to insert a break tag in the article itself. Use the Teaser Break button in your editor or turn off rich-text and manually insert a <!--break--> tag where appropriate.

Editor Teaser Break Button: Put cursor where you want then click the teaser break button.