what is indexing? how many types of indexing?

Your question is too much theoritical to explain here. It is beyond the scope of this discyussion. Kindly follow a good book for the same.

let me simply difine what is index... index is a virtual serial number given to the columns in a table to keep them in order..........

hi Ashmin,
Indexes are used on the table ir created on the table to make the data manupulation on the table faster.it is same as indexex on our books which helps to find the lession faster.
There are two types of indexes clustered index and nonclusterd index.There can be only one cluster index on a table.

Most database administrators are familiar with the potential performance benefits they can gain through the judicious use of indexes on database tables. Indexes allow you to speed query performance on commonly used columns and improve the overall processing speed of your database.

Microsoft SQL Server supports two types of indexes: Clustered indexes define the physical sorting of a database table’s rows in the storage media. For this reason, each database table may have onlyone clustered index. If a PRIMARY KEY constraint is created for a database table and no clustered index currently exists for that table, SQL Server automatically creates a clustered index on the primary key. Non-clustered indexes are created outside of the database table and contain a sorted list of references to the table itself.

SQL Server 2000 supports a maximum of 249 non-clustered indexes per table.
However, it’s important to keep in mind that non-clustered indexes slow down
the data modification and insertion process, so indexes should be kept to minimum One of the hardest tasks facing database administrators is the selection of appropriate columns for non-clustered indexes. You should consider creating non-clustered indexes on any columns that are frequently referenced in the WHERE clauses of SQL statements. Other good candidates are columns referenced by JOIN and GROUP BY operations.