In general, the web crawler starts with a list of URLs of the website to visit. As it visits these URLs, it identifies all the content and hyperlinks in the page and adds them to the list of URLs to visit. The process is either ended after a certain number of links has 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 user and therefore can make the site appear slow to the other users if they access the same site repeatedly. One command that web crawlers are supposed to obey is the robots.txt protocol, with which web site owners can indicate which pages should not be indexed. Also the Meta Robot tag can be implemented in the website code which indicates the crawl weather to index the website or not.
Thus the basic overview of web Crawler shows that it’s just an automated procedure that is followed to index the website links and URL’s.