Explain ACID properties in database transactions.

Atomicity: Transaction is all-or-nothing
Consistency: Database remains in valid state
Isolation: Concurrent transactions don't interfere
Durability: Committed changes are permanent

Example:
START TRANSACTION;
UPDATE accounts SET balance = balance - 100 WHERE id = 1;
UPDATE accounts SET balance = balance + 100 WHERE id = 2;
COMMIT; -- Both updates succeed or both fail