Final answer:
A Web crawler systematically visits every webpage on a site by starting from the homepage or other primary pages, then reading and following all hyperlinks on each page, while managing visited URLs and respecting web server constraints.
Step-by-step explanation:
A Web crawler, also known as a spider or bot, navigates the web to index the content of websites for search engines. To visit every web page at a website, crawlers start with a list of URLs to visit, called the seeds, which are usually the homepage or primary pages of a site. From these initial points, the crawler looks for hyperlinks to other pages on the same site and systematically access them.
The process involves the crawler reading the page's content and following all the links found within it. This procedure continues recursively, which allows the crawler to move from one page to another and index the entire website. To ensure that the crawler does not visit the same page multiple times, it keeps track of all the URLs it has already visited using a data structure referred to as the 'crawl frontier.'
The crawler must also respect certain rules set by websites, such as the instructions in a site's robots.txt file, which may restrict the crawler's access to specific web pages. Additionally, crawlers need to manage their visiting frequency and parallel connections to avoid overloading the web server, a concept known as crawl politeness.