Isolation Levels in SQL Server

An isolation level that defines the degree how one transaction must be isolated from resource or data modifications made by other transactions. Isolation levels are described as which concurrency side-effects, such as dirty reads or phantom reads, are allowed.

There are four type of Isolationlevel in SQL server
    Read uncommitted (lowest level)
    Read committed
    Repeatable read
    Serializable  (highest level)

Please refer

http://msdn.microsoft.com/en-us/library/ms378149%28v=sql.90%29.aspx

hope this will help you.