Drupal selects the first part of a node's text to use as an extract in indexes of nodes. You can manually control the length of the extract. In Drupal 6, you can also switch off
Show summary in full view to override the extract with manually entered data. The Excerpt module gave you similar control in Drupal 4. There may be additional features for Excerpt in Drupal 5 and 6.
I used the Excerpt module on some pages and let the system default to an automatic extract on other pages. On most pages I set the limit for the automatic extract using the <!--break--> element in the text. I may then edit the automatic extract to make the meaning clearer.
Drupal 7 breaks out the teaser into a separate field and makes the Excerpt module redundant.