Clustered index will be created implicitly when we have primary key column in the table.There we can have only one primary key. So There will be only one clustered index.

Non clustered index should be created explicitly. There we can have up to 249 non clustered index as per sql server 2005 and 999 as per the sql server 2008.

When compared with non clustered index, the clustered index will retrieve the result very quickly.Because the data will be stored on the same node of index key.
But in non clustered index the data will be on the separate page of the memory location and node will have the reference page of the data.

I think th emost basic difference is:
Clustered orders the table where as non-clustered does not.