The Lord called on Jonah to go to Ninevah and preach to them there. Jonah attempted to run from God's call. He took a ship to another place, or so he thought.
A great storm came up and Jonah ended up inside a fish for three days and nights. The fish brought him to shore, and again the Lord called Jonah to go and speak to the people of Ninevah.
So Jonah went to Ninevah and told the people there that in forty days their land would be overturned (if they didn't turn away from their evil actions). The Ninevites turned from their evil ways, fasted and repented. The Lord spared them.
Since the Ninevites at the time were enemies to Jonah's people, Jonah was not happy that they were spared.
In Jonah 4:11 the Lord speaks to Jonah about his hard heart; "But Nineveh has more than a hundred and twenty thousand people who cannot tell their right hand from their left, and many cattle as well. Should I not be concerned about that great city?"