A database Index is a data structure that improves the speed of operation on a database.

Index  can be classified as clustered or non-clustered.

1.Clustered Index

1. The leaf level of a clustered index is the actual
   data
2. the logical order of the index matches the physical stored
   order of the rows on disk.
3. There can be only one Clustered index for a table.
4. usually made on the primary key.

 2.Non-clustered Index

1. the leaf level is actually a pointer to the data in rows
2. the logical order of the index does not match the physical
   stored order of the rows on disk
3. There can be only 249 Clustered index for a table
4. usually made on the any key(Unique Key or any other)