The work of the search engine is divided into three stages, i.e., crawling, indexing, and retrieval.
1) Crawling
This is the first step in which a search engine uses web crawlers to find out the webpages on the World Wide Web. A web crawler is a program used by Google to make an index. It is designed for crawling, which is a process in which the crawler browses the web and stores the information about the webpages visited by it in the form of an index.
So, the search engines have the web crawlers or spiders to perform crawling, and the task of crawler is to visit a web page, read it, and follow the links to other web pages of the site. Each time the crawler visits a webpage, it makes a copy of the page and adds its URL to the index. After adding the URL, it regularly visits the sites like every month or two to look for updates or changes.
2) Indexing
We can say that the index is like a huge book which contains a copy of each web page found by the crawler. If any webpage changes, the crawler updates the book with new content.
So, the index comprises the URL of different webpages visited by the crawler and contains the information collected by the crawler. This information is used by search engines to provide the relevant answers to users for their queries. If a page is not added to the index, it will not be available to the users. Indexing is a continuous process; crawlers keep visiting websites to find out new data.
3) Retrieval
This is the final stage in which the search engine provides the most useful and relevant answers in a particular order in response to a search query submitted by the user. Search engines use algorithms to improve the search results so that only genuine information could reach the users, e.g., PageRank is a popular algorithm used by search engines. It shifts through the pages recorded in the index and shows those webpages on the first page of the results that it thinks are the best.