Above the main body field when creating content there is an option to edit the Summary. If this is not actively created the summary will be the first 600 characters of the piece of content. A summary is displayed as a 'Teaser' in certain circumstances. For example, if a piece of content is marked as 'Promoted to the front page' its Teaser or Summary will appear on the front page. If the full item is longer than the summary, a 'Read More' link to the main page for this piece of content will appear automatically. The link in the item's title will also lead to its own page where it is displayed in full. The summary or teaser view also appears on the News page where News Items are aggregated.
