Since version 2.9 WordPress has featured a trash bin. It contains any posts, pages, attachments and comments that have been deleted. By default the trash bin is emptied every 30 days. When you empty the trash bin it permanently deletes any items in the trash and the only way to recover them is to go to backup and restore them.
You can modify the interval by setting the following in your wp-config.php.
define( 'EMPTY_TRASH_DAYS', 30 ); // 30 days is the default
To disable trash set the number of days to zero. Note that WordPress will not ask for confirmation when someone clicks on “Delete Permanently”.
define( 'EMPTY_TRASH_DAYS', 0 ); // Zero days
A previous post discusses how to work with the wp-config.php file.