Isolation Level

• A degree to which one transaction is isolated from another transactions.
• Lower the transaction, increases concurrency and decreases data correctness.
• Total 5 type of Isolation Level
• READ UNCOMMITTED
• Lowest level 
• does not issue SHARED LOCK - means other transactions can see the data 
• are not blocked by EXCLUSIVE LOCK - means they can read uncommitted modifications, this is same as using NOLOCK
• This level produces "dirty read"
• READ COMMITTED
• Default level in SQL Server
• cannot read the data which has been modified but not committed
• it prevents "dirty read"
• it produces "non repeatable read"
• REPEATABLE READ
• contains features of READ COMMITTED+ cannot modify data that has been read by other transaction until it gets completed
• SERIALIZABLE
• strongest level
• contains features of REPEATABLE READ + cannot insert data if a table is in use by some other transaction (read/update)
• SNAPSHOT
• introduced in 2005
• here is the comparison of basic 4 types of isolation levels how it behaves differently for different DML Operation

		Session 1 (Within Transaction)		Session 2
				SELECT	UPDATE / DELETE	INSERT
READ UNCOMMITTED		SELECT		Yes	Yes	Yes
		UPDATE / DELETE		Yes	No	Yes
		INSERT		Yes	No	Yes
READ COMMITTED		SELECT		Yes	Yes	Yes
		UPDATE / DELETE		No	No	Yes
		INSERT		No	No	Yes
REPEATABLE READ		SELECT		Yes	No	Yes
		UPDATE		No	No	Yes
		INSERT		No	No	Yes
SERIALIZABLE		SELECT		Yes	No	No
		UPDATE		No	No	No
		INSERT		No	No	Yes

• SET TRANSACTION LEVEL REPEATABLE READ. This t-sql command sets different transactional level
• only 1 isolation level can be set at any point of time
• Any isolation level can be switched to any isolation level during a transaction (exception: SNAPSHOT)
• If isolation level is set inside stored procedure, then at the end of execution of SP, isolation level will be set to original what it was before execution of SP.

For more info about Locks

http://www.sqlteam.com/article/introduction-to-locking-in-sql-server