A city-state (what the Greeks called a polis, which is where our word politics comes from) is like a very small country, with just one city in it. There are still some city-states in the world today, like Monaco or Luxembourg. But in antiquity and the Middle Ages, city-states were very common.They might have any of a number of different forms of government.
The Standard of Ur, from West Asia (2000 BC)
The first known city-states were in West Asia, where there were many city-states throughout the Bronze Age, sometimes unified under a leader like Sargon of Akkad, and sometimes not. Uruk is one example of these Sumerian city-states. These city-states were ruled by kings, with councils of noblemen for advisors, as we see in the Epic of Gilgamesh.
Greek theater at Epidauros (200 BC)
Greece, in the Bronze Age, also was organized into many small city-states, which are listed in Homer's Iliad: Mycenae, Sparta, Pylos, Athens, Corinth, Ithaca, and so on. These city-states also had kings.
At the beginning of the Iron Age, many different people
made new city-states all around the Mediterranean Sea: the Etruscans,
the Romans, the Greeks,
and the Phoenicians. Most
of these were ruled by oligarchies or democracies.