Trash is a new functionality from WordPress 3.0. When an item (post, comment, etc) is deleted, it goes in Trash instead of being permanently removed. Here is a tip to tell your WordPress blog to autmatically empty trash every X days.
Simply open your wp-config.php file (located at the root of your WordPress install) and paste the following code:
define('EMPTY_TRASH_DAYS', 10 );
The second parameter is when to empty trash, in days.