Search engines have varying mechanisms that make them work, but they generally involve three functions: searching the Web, listing important information they find in an index, and providing search query results based on that index. They rely on algorithms and rankings to provide relevant results.
To search the Web, a search engine employs automated programs known as spiders. Starting with popular websites, these spiders crawl Web pages and search for significant words. They find and follow links in each page to explore more pages. To manipulate the behavior of these spiders, website owners can use meta tags to describe their pages with specific keywords. Website owners can also add robot exclusion protocols to signal the spiders to ignore specific pages.
After crawling, the spiders submit their findings in the search engine's index. Besides listing words and the URLs that contain these words, a search engine may assign a weight to every entry in its index. An entry's weight determines how often the word is used on a page and influences the entry's value in the index. Algorithms are also used to determine the rankings of entries in the index. For example, Google's PageRank algorithm values pages that are frequently linked to by other pages.
When a user submits a query, the search engine returns the results it deems the most relevant. The user can refine the results further through Boolean operators.