This plug-in, does one very simple job. It adds an extra field called Excerpt Length to the Reading Settings section of the WordPress admin.
This is value is used to specify the number of words that you would like to appear in the the_excerpt().
It doesn’t require you to add any extra PHP code to your theme’s functions.php, nor does it mean that you have to edit the formatting.php in the core! (As a general rule… never, ever edit any of the core WordPress files – as it makes upgrading painful!)