Explain the different transaction isolation levels.

Isolation levels define visibility of uncommitted changes: READ UNCOMMITTED (dirty reads allowed - sees uncommitted data), READ COMMITTED (only sees committed data - default in many DBMS), REPEATABLE READ (same query returns same results within transaction), SERIALIZABLE (highest isolation - transactions appear sequential). Higher isolation prevents more anomalies (dirty/non-repeatable reads, phantom rows) but reduces concurrency. Trade-off between consistency and performance.