In Clustered Index, the data is physically sorted.
only one Clustered Index is created per table.
The data is stored in heap level of the B-tree.

In non-clustered Index, the physical order of the rows is 
not same as the order of index.
there can be upto 249 non-clustered index per table.
the data is present in the random order but the logical 
ordering is specified by the index.

The difference between a clustered index and a non-
clustered index is that when you use a clustered index, the 
section of the table that comprises the index has its data 
pages linked differently from those data pages comprising a 
non-clustered index.

while creating primary key it will create cluster index.
while creating unique key it will create noncluster index
---cluster index it will create only one per table
---Non Cluster index it will create up to 249 per table
--cluster index physical sort the data
--Non cluster index logical sort the data.