What is the difference between clustered and nonclustered indexes?
There are clustered and nonclustered indexes. A clustered index is a
special type of index that reorders the way records in the table are
physically stored. Therefore table can have only one clustered index.
The leaf nodes of a clustered index contain the data pages.
A nonclustered index is a special type of index in which the logical
order of the index does not match the physical stored order of the
rows on disk. The leaf node of a nonclustered index does not consist
of the data pages. Instead, the leaf nodes contain index rows.
See this article fot more details:
Index Optimization Tips