An image saying everything about transaction isolation levels in SQL Server:
|Description||Dirty reads||Non-repeatable reads||Phantom reads|
|Read Uncommited||(S) locks are not acquired by SELECT statements.
You can also achieve this degree of isolation on a query basis using the NOLOCK locking hint:
SELECT * FROM dbo.TheTable WITH(NOLOCK);
|Read Commited||(S) locks are requested by the SELECT statements, but (S) locks are acquired only while data is read (not until the end of the transaction).||No||Yes||Yes|
|Repeatable Read||Shared locks are retained by the SELECT statements until the end of the transaction which completely prevents others from updating any data covered by the lock.||No||No||Yes|
|Serializable||Instead of acquiring a lock only on the row to be accessed, the Serializable isolation level acquires a range lock on the row and the next row in the order of the data set requested.
This prevents update and the addition of rows by other transactions in the data set operated on by the first transaction.