If a user is looking for information in a document set, the system searches the corresponding index rather than the documents themselves. An index is a data structure that allows you to find information efficiently. Searching in an index is considerably faster than searching directly in documents.
Below is a simplified example of how an index can be structured. The example shows an inverted index. This is used for an exact search. The inverted index consists of a list of terms that are extracted from documents. Each term references the document it appears in.
bus →document_1, document_5, document_4
bush → document_16, document_21
business → document_16, document_21
busy → document_16, document_21