Isolation Level controls the locking and row versioning 
behavior of Transact-SQL statements issued by a connection 
to SQL Server.

SET TRANSACTION ISOLATION LEVEL
    { READ UNCOMMITTED
    | READ COMMITTED
    | REPEATABLE READ
    | SNAPSHOT
    | SERIALIZABLE
    }
READ UNCOMMITTED: Specifies that statements can read rows 
that have been modified by other transactions but not yet 
committed. This option has the same effect as setting 
NOLOCK on all tables in all SELECT statements in a 
transaction.

READ COMMITTED: Specifies that statements cannot read data 
that has been modified but not committed by other 
transactions. This prevents dirty reads. When the 
READ_COMMITTED_SNAPSHOT database option is ON, you can use 
the READCOMMITTEDLOCK table hint to request shared locking 
instead of row versioning for individual statements in 
transactions running at the READ_COMMITTED isolation level.

REPEATABLE READ: Specifies that statements cannot read data 
that has been modified but not yet committed by other 
transactions and that no other transactions can modify data 
that has been read by the current transaction until the 
current transaction completes.

SNAPSHOT: Specifies that data read by any statement in a 
transaction will be the transactionally consistent version 
of the data that existed at the start of the transaction. 
The transaction can only recognize data modifications that 
were committed before the start of the transaction. A 
transaction running under SNAPSHOT isolation level can view 
changes made by that transaction. For example, if the 
transaction performs an UPDATE on a table and then issues a 
SELECT statement against the same table, the modified data 
will be included in the result set.

SERIALIZABLE: Specifies the following:

Statements cannot read data that has been modified but not 
yet committed by other transactions.

No other transactions can modify data that has been read by 
the current transaction until the current transaction 
completes.

Other transactions cannot insert new rows with key values 
that would fall in the range of keys read by any statements 
in the current transaction until the current transaction 
completes.

USE AdventureWorks;
GO
SET TRANSACTION ISOLATION LEVEL REPEATABLE READ;
GO
BEGIN TRANSACTION;
GO
SELECT * 
    FROM HumanResources.EmployeePayHistory;
GO
SELECT * 
    FROM HumanResources.Department;
GO
COMMIT TRANSACTION;
GO