Hello Julia!

"Város" for example: Budapest, London, Moscow etc.

"Belváros" : The city center where a lot of people. Many things can be done. (downtown)

"Külváros" : The outer parts of the city. There are usually fewer people live than in the city center. (suburb)

I speak English badly, but I hope I helped :).

Belváros is downrown or insid of the town. külváros is periferia of the town.

Thank you very much!!!!