What are the transaction isolation levels?

There are four transaction isolation levels: 
1. READ UNCOMMITTED: A transaction may read any version of data, committed or not. This is achieved in a locking implementation by read requests proceeding without acquiring any locks.
2. READ COMMITTED: A transaction may read any committed version of data. Repeated reads of an object may result in different (committed) versions. This is achieved by read requests acquiring a read lock before accessing an object, and unlocking it immediately after access.
3. REPEATABLE READ: A transaction will read only one version of committed data; once the transaction reads an object, it will always read the same version of that object. This is achieved by read requests acquiring a read lock before accessing an object, and holding the lock until end-of-transaction.
4. SERIALIZABLE: Fully serializable access is guaranteed.