The Idea of Indexing in NLP for Information Retrieval

Did you ever wondered how an Information Retrieval occur when you search for a query on Google or any other search engine?

Information Retrieval can be defined as finding material of an unstructured nature that satisfies the information need from within large collections.

In the 90's, the searching for an information on the Internet is less memory consuming because at that time there is less document available online and that is in structured manner on the Internet, but as the Internet usage...