Search Engine Crawler's

Search engine crawls the website with their crawler for the data to present to their search quires put across by the visitors , the crawls search all the web pages in the website to create a quick response to the query put across by the visitor through the search engine like google ,yahoo, MSN, Bing, Ask, Alta vista Etc.When every new web page or content is uploaded to the website the crawlers immediately crawls the website and stores in it data base , so to make sure the visitors gets the right information in quick time

Here are some of the search engine and their search crawlers name:
Google- googlebot
Yahoo-Slurp
MSN-MSNbot

For More reference Check this link out
http://en.wikipedia.org/wiki/Web_crawler