An index is used when we need a bookmark for our table. They are just used to speed up searches/queries.
A clustered index physically sorts the data in a table.The value of a clustered index is a key-value pair,where key is the index key and value is the actual value.
Non- Clustered Index:
A non-clustered index sorts the data logically but not physically.The value of a non-clustered index is not the data but a pointer to the data page.So, we can say a non-clustered index is dependent on the clustered index.In the absence of clustered index, it refers a physical location in the Heap for value.