A Web crawler is a computer program that browses the World Wide Web in a methodical, automated manner.
This process is called Web crawling or spidering. Many sites, in particular search engines, 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. Also, crawlers can be used to gather specific types of information from Web pages, such as harvesting e-mail addresses (usually for spam).
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.
A vast amount of Web pages lie in the deep or invisible Web. These pages are typically only accessible by submitting queries to a database, and regular crawlers are unable to find these pages if there are no links that point to them.
Deep Web crawling also multiplies the number of Web links to be crawled. Some crawlers only take some of the <a href="URL" shaped URLS. In some cases, such as the Googlebot, Web crawling is done on all text contained inside the hypertext content, tags, or text.
Bget is a powerful, fast and professional web data extraction software that will extract the data from any type of websites through flexible rules, therefore what you can see through the browser is also what you can get.