How do search engines work?

Search engines utilize algorithms to index and crawl the internet, classifying online sites according to popularity and relevancy. The search engine collects relevant web pages from its index in response to a user's query. It ranks them according to several criteria, including keyword density, backlinks and user engagement metrics. 
The user is then shown the search engine's results in order of perceived relevancy. Search engines regularly upgrade their algorithms to prevent spam and increase the accuracy of their results.