An index is a table structure that SQL Server uses to provide fast access to rows of a table based on the values of one or more columns. An index contains data values and pointers to the rows where those values occur.
In the following illustration, for example, the employee table in the pubs database has an index on the employee ID (emp_id) column.
SQL Server uses an index as you use the index of a book. SQL Server searches the index to find a particular value and then follows the pointer to locate the row containing the value. For example, in the preceeding illustration, SQL Server locates the record containing "VPA30890F " by searching the index for "VPA30890F."
For details on creating indexes, see Using Indexes.