Search engines use web crawlers to crawl websites. Web crawlers, also known as spiders or bots, are automated programs that visit websites and download their content. They then follow links on those pages to find more pages to download. This process is called crawling.

Once a web crawler has downloaded a web page, it extracts the text and other content from the page and stores it in a database. This database is called the index. When a user performs a search on a search engine, the search engine retrieves the relevant pages from its index and ranks them according to a number of factors, including the content of the pages, the number of links to the pages, and the quality of the links.

More detailed explanation of how web crawlers work:

1. The web crawler starts with a list of URLs, which it can obtain from a variety of sources, such as its own index of known websites, sitemaps submitted by webmasters, and links from other websites.
2. The web crawler visits each URL on its list and downloads the content of the page.
3. The web crawler extracts the text and other content from the page, such as images, videos, and CSS files.
4. The web crawler stores the extracted content in its database.
5. The web crawler follows links on the page to find more pages to download.

Web crawlers are constantly crawling the web to discover new and updated content. The frequency with which a web crawler crawls a particular website depends on a number of factors, such as the size and popularity of the website.

Here are some of the technologies that web crawlers use:

• HTTP: Web crawlers use the HTTP protocol to communicate with web servers and download web pages.
• HTML: Web crawlers use the HTML markup language to parse web pages and extract the content.
• CSS: Web crawlers can use CSS to understand the structure and style of web pages.
• JavaScript: Web crawlers can execute JavaScript on web pages to extract content that is generated dynamically.
• Robots.txt: Web crawlers follow the robots.txt file to determine which pages on a website they are allowed to crawl.
• Sitemaps: Web crawlers can use sitemaps to discover all of the pages on a website.

Web crawling is an essential part of how search engines work. By crawling the web and indexing its content, search engines are able to provide users with access to the vast amount of information that is available online.