What is the use of indexes and what are the types of indexes available in SQL Server?

What is the use of indexes and what are the types of indexes available in SQL Server?
Indexes are used to find data quickly when a query is processed in any relational database. Indexes improve performance by 10 to 500 times.
Index can improve the performance in following operations:

* Find the records matching with WHERE clause

UPDATE Books SET Availability = 1 WHERE SubjectId =12
DELETE FROM Books WHERE Price <10
SELECT * FROM Books WHERE Price BETWEEN 50 AND 80
* Sorting the result with ORDER BY

SELECT * FROM Books ORDER BY Price DESC
* Grouping records and aggregate values

SELECT Count(*) as Units, Price FROM Books GROUP BY Price

There are two types of indexes available in SQL Server: clustered and non-clustered
Clustered index
Clustered index physically reorders the records of a table. Therefore a table can have only one clustered index. Usually a clustered index will be created on the primary key of a table.
Non – Clustered Index
Non – Clustered index are stored in the order of the index key values, but the information in the table is stored in a different order. Means logical sorting of data not Physical. In SQl Server 2005 a table can have 249 non-clustered indexes.
Composite Indexes
A composite index is an index on two or more columns. Both clustered and non-clustered indexes can be composite indexes. If you have composite index on Price and BookName then take can take advantage of it like this:
SELECT BookName, Price FROM Products ORDER BY UnitPrice BookName, Price DESC