Isolation Levels in SQL Server

An isolation level determines the degree of isolation of data between concurrent transactions.
4 levels
1.Read committed(default)
Any shared locks you create will be automatically released as soon as the statement that created on them is complete.But with this isolation level unrepeatable reads and phanthoms can still occur.
2.Read uncommitted
Setting isolation level to read uncommitted tells the sql server not to set any locks and not to honour any locks but it gives highest performance interns of speed.
3.Repeatable read
It escalatesyour isolation level and provides an extra level of concurrency ptotection by preventing not only dirty reads but also preventing unrepeatable reads.
4.Serialazable
It prevents all forms of concurrency issues except for a lost update.Even Phanthoms are also prevented.