Web crawler or a spider is a computer program that browses the World Wide Web in a systamatic and automated manner. Other terms for Web crawlers are ants, automatic indexers, bots or Web spider, Web robot.
Many sites, specially search engines, like google, yahoo, askjeeves use spidering as a means of providing up-to-date data. Web crawlers are mainly used to create a copy of all the visited pages for later processing by a search engine that will index the downloaded pages to provide fast searches. Crawlers can also be used for automating maintenance tasks on a Web site, such as checking links or validating HTML code.
A Web crawler is one type of bot, or software agent. In general, it starts with a list of URLs to visit, called the seeds. As the crawler visits these URLs, it identifies all the hyperlinks in the page and adds them to the list of URLs to visit, called the crawl frontier. URLs from the frontier are recursively visited according to a set of policies.
In simple search engine terms, a “spider” is an electronic robot that travels the web examining websites in order to add them to a search engine database and rank them according to the specific ranking criteria for that search engine. Most search engines use spiders to quickly and efficiently canvass the web for information.