COMMIT keeps the changes of a transaction. ROLLBACK throws them away.

ROLLBACK ends the transaction. Every row is the same as before BEGIN. Other sessions never saw the changes.

A safety net for risky changes

Chapter 6 showed what an UPDATE without WHERE does: it changes every row. Inside a transaction, you can still undo it:

BEGIN;
UPDATE accounts SET balance = 0;   -- the WHERE is missing
SELECT * FROM accounts;            -- every balance is 0
ROLLBACK;                          -- every balance is back

Use this order for any change that you are not sure about:

1. BEGIN;
2. The change.
3. A SELECT to check the rows.
4. COMMIT; if the rows are correct, ROLLBACK; if they are not.

After COMMIT, it is too late

ROLLBACK undoes only the open transaction. After COMMIT, there is no open transaction:

WARNING:  there is no transaction in progress

The committed change stays. To undo it, you must write a new statement that changes the rows back.

Tables too

In PostgreSQL, CREATE TABLE, ALTER TABLE and DROP TABLE are also part of the transaction. ROLLBACK undoes them too.