By default Drupal will trim your posts to 600 characters. What this means is that if you have a story or posting that is longer than 600 words, Drupal will cut the content to the first 600 characters for the front page and then add a 'read more' link to the full posting.
There is a setting in Drupal that allows you change the ammount of characters that it will trim to. You can set it to the maximum of unlimited, meaning that your whole post will show on the front page and it won't cut any characters, to a minimum of 200 characters. If you set it to the mimimum, it would be like having the first few sentences of your post on the front page.