There are six different "flavors" of city-states. Each can provide you with different benefits if you befriend or ally with them.
A trade city-state can provide Gold to your civilization.
A Religious city-state can provide Faith to your civilization
A militaristic city-state can provide Production toward producing units.
A scientific city-state can provide Science to your civilization.
A cultural city-state can help you improve your Culture.
An industrial city-state can provide Production towards wonders, buildings, and districts.