Create a sample table on a sample database

CREATE TABLE [dbo].[Table_1](
    [Id] [int] IDENTITY(1,1) NOT NULL,
    [title] [varchar](50) NULL,
 CONSTRAINT [PK_Table_1] PRIMARY KEY CLUSTERED 
(
    [Id] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]

Now open a First query editor (on the database) insert the code below, and execute (do not touch the --rollback) in this case you insert a row on DB but do not commit changes.

begin tran

INSERT INTO Table_1 values('Title 1')

SELECT * FROM [Test].[dbo].[Table_1]

--rollback

Now open a Second Query Editor (on the database), insert the code below and execute

begin tran

set transaction isolation level READ UNCOMMITTED

SELECT * FROM [Test].[dbo].[Table_1]

You may notice that on second editor you can see the newly created row (but not committed) from first transaction. On first editor execute the rollback (select the rollback word and execute).

-- Rollback the first transaction
rollback

Execute the query on second editor and you see that the record disappear (phantom read), this occurs because you tell, to the 2nd transaction to get all rows, also the uncommitteds.

This occurs when you change the isolation level with

set transaction isolation level READ UNCOMMITTED

Remarks

You can read the various ISOLATION LEVEL on MSDN (opens new window)