It's the program that search engines send out to find and read sites, also known as a spider or bot. No crawlers, no search engines, no seo.
For more information on how they work, exactly, look at Google's Webmaster Essentials page (go to google webmaster tools). There's plenty of good stuff there for you...
Crawler; a person who tries to please someone in order to gain a personal advantage Crawler; a person who crawls or creeps along the ground Crawler; another name for a news ticker Alright enough fun and games. I think you have the concept of a crawler wrong, a crawler or a search engine bot is the work of search engines. Crawlers will scan the internet to add web pages to a search engines index. You can learn a lot more by reading: http://en.wikipedia.org/wiki/Web_crawler ...and yes my 3 initial responses to a crawler are also correct