A clustered index is an index whose leaf nodes, that is the lowest level of the index, contain the actual data pages of the underlying table.
By default, a primary key constraint creates a clustered index.
In a nonclustered index, the lowest level of the index does not contain the data page of the table. Instead, it contains the information that allows SQL Server to navigate to the data pages it needs. For tables that have a clustered index, the leaf node of the nonclustered index contains the clustered index keys. In the previous example, the leaf node of a nonclustered index on the Customers table would contain the Customer_ID key.
If the underlying table does not have a clustered index (this data structure is known as a heap), the leaf node of the nonclustered index contains a row locator to the heap data pages.
