A freshly-installed copy of Drupal core includes the handy ability to generate short "teaser" text from a long node automatically. You can choose the character count that will be used, but unfortunately that's all: if you need to break on word boundaries or want to make it clear that the text is a truncated version of the full article, you're out of luck. If you need more flexibility, the Smart Trim module can help.
Smart Trim adds a new set of formatting options to Drupal's Text, Long Text, and Long Text With Summary field types. Like the existing Summary formatter, it allows you to choose the truncation length -- but you can also truncate at a specific number of words, ensuring that the break won't fall awkwardly between words. An elipses can be inserted automatically at the end of the truncated text, and HTML can be stripped out to ensure that unclosed tags don't sneak into teasers. It's even possible to automatically add a "Read more…" link that points to the full view of the node itself. While that one's less useful on node teasers, it can be handy when you're using the truncated summary text in a highly streamlined view; it eliminates the need to add a separate field to contain a link to the node.
Most of the options in Smart Trim are available in older modules for Drupal 5 and 6, or can be accomplished with a few snippets of custom code tucked in template files. This module collects all of them in one simple field formatter, however, and ensures the busy admins only need to grab one tool to make sure their teasers work just so.