What are different transaction levels in SQL SERVER?

The Transaction Isolation level decides how is the one process isolated from theother process. Using the transaction levels you can implement locking in the SQL SERVER.

There are 4 transaction levels in SQL SERVER as shown below:-

READ COMMITTED:

In this the shared lock is held for the duration of the transaction, means that no other transactions can modify the data at the same time. The Other transactions can insert and change data in the same table, however, as long as it is not locked by the first transaction.

READ UNCOMMITTED:

In this no shared locks and no exclusive locks are honored. This is the least restrictive isolation level resulting in the best concurrency but the least data integrity.

REPEATABLE READ:

In this the setting disallows dirty and non-repeatable reads. However, even though the locks are held on read data, the new rows can still be inserted in the table, and will subsequently be interpreted by the transaction.

SERIALIZABLE:

This is the most restrictive setting holding shared locks on the range of data. This  does not allow the insertion of new rows in the range that is locked; hence, no phantoms are allowed.