Index - Non-Clustered Index - similar to index of a text book's table of content which is always in separate page, similarly non clustered index creates sperate table to store the indexing

 Non Clustered Index :

similar to index of a text book's table of content which is always in separate page, similarly non clustered index creates sperate table to store the indexing 

one table can have more than one non-clustered index unlike clustered index.

NOTE: If the index key is NAME and if the query is around coumn Name then only the index table is used and has faster retrieval but if the querry is not name, or is salary or depratment id, the querry engine doesnt use index table and does TABLE SCAN

Note: Clustered index is superior to non clustered index.