How do search engines index web pages?

Search engines index web pages by using web crawlers to find, scan, and store information from websites.

Web crawlers, also known as spiders or bots, are automated software applications used by search engines to scour the internet, visiting websites to collect information. This process is known as web crawling or spidering. The information gathered by these crawlers is used to create an index, which is essentially a giant catalogue of all the web pages and their content found on the internet.

The web crawler starts with a list of URLs to visit, known as the seed. As the crawler visits these URLs, it identifies all the hyperlinks in the page and adds them to the list of URLs to visit. This process continues, with the crawler hopping from page to page via hyperlinks. The crawler also revisits websites periodically to check for any changes or updates to the content.

When a web crawler visits a page, it scans the content, noting key elements such as the page title, keywords, and links. It also looks at the HTML code to understand the structure of the site. This information is then used to create a map of the site, which is stored in the search engine's index.

The index is a massive database where all the information collected by the web crawlers is stored. When a user performs a search, the search engine doesn't scour the entire internet; instead, it looks at its index to find relevant pages. The search engine uses complex algorithms to determine the relevance of a page to the user's query, taking into account factors such as the location of the keywords on the page, the number of inbound and outbound links, and the overall authority of the page.

In addition to indexing, search engines also rank web pages based on their relevance and quality. This process, known as search engine optimisation (SEO), involves analysing various factors such as the use of keywords, the quality of content, and the number of links to a page. SEO is crucial for websites as it determines their visibility in search engine results, which can significantly impact the amount of traffic they receive.