There are clustered indexes and non-clustered indexes in SQL Server. With non-clustered indexes, the leaf level nodes contain only the index data with a pointer to the associated data page where the remaining data resides. As a result, data access that uses a non-clustered index may cause extra reads of the data from the data page. With clustered indexes, the leaf level nodes of the B-tree contain the actual data rows for the table. There is only one clustered index per table. Remember that the clustering key is used in all non-clustered indexes as the row identifier, so choose them wisely.