Changing the Full Post Content to an Excerpt :

WordPress makes changing your content displays on your own full version WordPress easy. To change the way your content displays on these pages, edit the appropriate template files with the built-in Theme Editor (Presentation > Theme Editor) or any text editor and search for the_content() and change it to the_excerpt(). That’s it.

The multi-post displaying template files to change may include:

■index.php
■archives.php
■archive.php
■category.php
■search.php

Do NOT change it in your single.php or page.php, if you have them in your Theme, as that will force only the excerpt and not the full content to show up on these single post views of your blog. Not a good thing.

By default, WordPress shows only the first 120 words or so of a post as the excerpt. It also strips out all links, styles, and tags so only the text appears. So if your excerpt doesn’t exactly match the look of the top part of your post, this is why.