There are little automated programs called web crawlers or robots that are sent out to crawl web pages for inclusion into the databases that make up search engines. Some of these bots are beneficial such as Googlebot, the robot that scours the Internet on behalf of the search engines of all search engines, Google.
Most of these bots are harmless, hardly making any waves during their routine visits. But then there are bots such as TWICELER!!! This bot causes havoc with pages that are dynamically generated by php scripts such as calendars and forums. It has been known to dramatically increase the bandwidth that a web site uses. It can even cause server resources to be used up to the point that it adversely effects the server and all the accounts on it.