If a Non-Clustered index is built over a Heap table or view (read more about SQL Server indexed views, that have no Clustered indexes) the leaf level nodes of that index hold the index key values and Row ID (RID) pointers to the location of the rows in the heap table. The RID consists of the file identifier, the data page number, and the number of rows on that data page.

On the other hand, if a Non-clustered index is created over a Clustered table, the leaf level nodes of that index contain Non-clustered index key values and clustering keys for the base table, that are the locations of the rows in the Clustered index data pages. 

A RID Lookup operation is performed to retrieve the rest of columns that are not available in the index from the heap table based on the ID of each row.

A Key Lookup operation is performed to retrieve the rest of columns that are not available in the index from the Clustered index, based on the Clustered key of each row,