A game in development.
Currently you can use it as a very inacurrate population simulator.
A structure is something that you can build on the map.
P.S. The building function is not implemented.
A city is a structure.
It's, well, a city / town / village / etc.
It consists of city components.
It's currently the only structure.
(And the only structure that people can live in, theoretically.)
Plot of land in city where you do specific things.
Such as farms, houses, factories, etc.