A database index is a data structure which use to improve the speed of operations on a database table.
It's a physical structure containing pointers to the data.
Index can be created using one or more columns of a database table.
Index are created in an existing table to quickly and efficiently lookups.
It is possible to create an index on one or more columns of a table, and each index is given a name.
There are two type of index available.
Clustered index :
Clustered index are physical form of sorting. Therefore only one clustered index can be created on each table because the data rows themselves can only be sorted in one order.
Non Clustered index :
Non clustered indexes are logical form of sorting. Therefore more than one Non clustered indexes can be created on each table.