What are dirty reads, non-repeatable reads, and phantom reads?

Dirty read: Transaction A reads data that Transaction B has modified but not yet committed. If B rolls back, A read invalid data. Prevented by Read Committed+. Non-repeatable read: Transaction A reads a row, Transaction B updates and commits it, then A reads the same row again and gets a different value. The same query returns different results within a transaction. Prevented by Repeatable Read+. Phantom read: Transaction A queries rows matching a condition, Transaction B inserts a new matching row and commits, then A re-runs the query and sees a new "phantom" row. Prevented only at Serializable level. Each requires progressively higher isolation to prevent.