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.
clustered index is physically stored
a table can have 1 clustered index
non clustered index is logically stored
a table can have 249 non clustred index