In databases, indexes are special structures used to quickly locate certain rows. Indexes are most often used when you run SQL queries (SELECT statements) that look for rows matching given criteria.

Unlike tables or views, indexes can't be seen or browsed by database users. You will only see them "in action" when you run queries on large amounts of data; when properly used, indexes will make those queries faster.

A database index is similar to the index in the back of a book. A book index is organized to help you find the information you're looking for; you'll locate that quote much quicker because the index uses keywords that are sorted alphabetically. You don't have to read every single page to reach the right part of the book – you can simply check the index and turn to the relevant part.