uery_posts() is a way to alter the main query that WordPress uses to display posts. It does this by putting the main query to one side, and replacing it with a new query. To clean up after a call to query_posts, make a call to wp_reset_query(), and the original main query will be restored.
It should be noted that using this to replace the main query on a page can increase page loading times, in worst case scenarios more than doubling the amount of work needed or more. While easy to use, the function is also prone to confusion and problems later on. See the note further below on caveats for details.