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A database index is a data structure that is created to provide a fast and efficient means of finding data rows in tables.  You can picture this the same way an index of a book helps you find specific material quickly. 

In a relational database, an index is a copy of one part of a table. Indices can be created using one or more columns of a database table.  However, a simplified visualization of an index would be to picture it as a separate table with one column containing a key value and a second column containing a pointer to (or the location of) the row within a table containing the same key value. The index entries are maintained in key sequence.  This provides for faster querying using various types of search technique that deliver non-linear response times. Some databases also allow indices to be created only for records that satisfy some conditional expression.

Some fields are automatically indexed by the DBMS.  Primary keys and fields marked as ‘unique’ fall into this category.  This is done to aid in the efficiency and quick enforcement of constraints.  If a field such as social security number or email address is marked as ‘unique’ then the database will create and use the index to ensure that data is not introduced which conflicts with this constraint.

There are drawbacks to database indices as well.  Database indices take up storage space.  They also make for slower data insert, updates, and deletes since both the data within the tables and the indices need to be updated.