A web crawler (also known as web spider) is a program which browses the World Wide Web in a methodical, automated manner. A web crawler is one type of bot. Web crawlers not only keep a copy of all the visited pages for later processing - for example by a search engine but also index these pages to make the search narrower.
In general, the web crawler starts with a list of URLs to visit. As it visits these URLs, it identifies all the hyperlinks in the page and adds them to the list of URLs to visit. The process is either ended manually, or after a certain number of links have been followed.
Web crawlers typically take great care to spread their visits to a particular site over a period of time, because they access many more pages than the normal (human) user and therefore can make the site appear slow to the other users if they access the same site repeatedly.
For similar reasons, web crawlers are supposed to obey the robots.txt protocol, with which web site owners can indicate which pages should not be spidered.
The procedure of following links and not submitting queries to databases causes much content to be ignored: the deep web.
InternetAdSales.com: Robots, Spiders, Crawlers and HTTP_User_Agents (http://www.internetadsales.com/modules/wfsection/index.php?category=23) - Comprehensive listing of common web crawlers
Google Dance Tool (http://www.google-dance-tool.com/) - A tool to help webmasters determine when Google's webcrawler is crawling the web
Searchengines were also known as some of the brightest stars in the Internet investing frenzy that occurred in the late 1990s.
Most Web searchengines are commercial ventures supported by advertising revenue and, as a result, some employ the controversial practice of allowing advertisers to pay money to have their listings ranked higher in search results.
A recent enhancement to searchengine technology is the addition of geocoding and geoparsing to the processing of the ingested documents being indexed, to enable searching within a specified locality (or region).
From the searchengine's point of view, there is a cost associated with not detecting an event, and thus having an outdated copy of a resource.
Recently commercial searchengines like Ask Jeeves, MSN and Yahoo are able to use an extra "Crawl-delay:" parameter in the robots.txt file to indicate the number of seconds to delay between requests.
DataparkSearch is a crawler and searchengine released under the GNU General Public License.
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