By using software a website is crawled called a spider in the case of Google. During this the crawler will looks for links to different pages from that website and look at the text on a website. The spider can crawl webpages more then one before they gets indexed and the spider will start from URLs found on previous indexed pages. A web crawler does not visit every page daily as this would be not needed when some pages have been never change and the crawler visit pages with frequent changes.