So the people of Nineveh believed God, and proclaimed a fast, and put on sackcloth, from the greatest of them even to the least of them.
So the people of Nineveh believed God - gave credit to Jonah's message from God; thus recognizing Yahweh as the true God. Literally, 'believed IN God,' which expresses more than "believed God." They not only believed His word as true, but believed in Himself, trusting in Him. However they came to know Jonah's history, he was a sign to them at once of wrath, if they should disregard the message from God, and mercy, if they should regard it.
And proclaimed a fast, and put on sackcloth. In the East outward actions are often used as symbolical expressions of inward feelings. So fasting and clothing in sackcloth were customary in humiliation. Compare in Ahab's case, parallel to that of Nineveh, both receiving a respite on penitence (1 Kings 21:27; 1 Kings 20:31-32; Joel 1:13).
From the greatest of them even to the least - the penitence was not partial, but pervading all classes.