SQL Server isolation levels:

READ COMMITTED: Shared locks are held while any data is being read.

READ UNCOMMITTED: Specifies isolation level 0 locking. There are thus no shared locks or exclusive locks. It is the least restrictive of all the isolation levels.

REPEATABLE READ: Locks are applied on all data being used by a query. However, new phantom rows can be inserted into the data set by another user and are included in later reads within the current transaction.

SERIALIZABLE: Issues a range lock on data set, preventing other users to update or insert data into dataset until the transaction is complete.