What are MySQL transaction isolation levels?

READ UNCOMMITTED: Allows dirty reads
READ COMMITTED: Prevents dirty reads
REPEATABLE READ: Prevents dirty and non-repeatable reads (MySQL default)
SERIALIZABLE: Highest isolation, prevents all phenomena

Example:
SET SESSION TRANSACTION ISOLATION LEVEL READ COMMITTED;
START TRANSACTION;
SELECT * FROM accounts WHERE id = 1;
-- Other transactions can't modify this row
COMMIT;