Clustered Index: Clustered index defines the way in which data is ordered physically on the disk. And there can only be one way in which you can order the data physically. Hence there can only be one clustered index per table.
Why care about clustered index? If we put clustered index on the table then the data is retrieved much more faster because sql server doesnot have to read the whole data -- Depends on query. But data retrieval is much more faster.
NOTE: Although you can create more than one Non-Clustered index on a single table.
