The difference is that, Clustered index is unique for any
given table and we can have only one clustered index on a
table. The leaf level of a clustered index is the actual
data and the data is resorted in case of clustered index.
Whereas in case of non-clustered index the leaf level is
actually a pointer to the data in rows so we can have as
many non-clustered indexes as we can on the db.

I am explaining you with an example, The Telephone 
Directory is a fine example of Clustered Index as data and 
index are at the same page, whereas index in the back side 
of the book is a fine example of non-clustered index and 
non-clustered index is a fast B-tree structure as index 
just points to the data page. Also only one clustered index 
is possible per table and 249 non-clustered index per table.