Indexes are special lookup tables that the database search engine can use to speed up data retrieval. They contain a pointer to each row in the table and allow the database to quickly locate rows matching a specific criteria. Think of an index in a book; it allows you to quickly find a specific page without having to read every page. Similarly, an index in a database allows the database to quickly find the rows that match a specific condition in a query. Indexes are crucial for large tables where scanning the entire table would be extremely slow. They are particularly useful for frequently queried columns. Creating an index involves defining a key or keys on which the index will be based. The database engine then builds a data structure that allows it to quickly locate rows based on the values in the index. Indexes are not stored in the same way as the table data, but rather as separate data structures. This means that they don't take up much space in the table itself, but they do require additional space for the index structure. However, the performance gains from using indexes often outweigh the small space overhead.