There are two differences. The first difference is the difference between cities and towns. Cities are a lot bigger than towns. A city could have skyscrapers, big monuments, or large museums. London, New York, and Tokyo are cities.
Towns are smaller than cities. A town can be very small or it can be big. Towns are places that you don't usually hear about unless something important happens there. Cities often have lots of towns surrounding them.
The other difference is the difference between the verbs. 'The city has been built.' This means that the builders are done working on the city. The city is finished. More might be added later, but all of the important buildings are there.
'The town is being built.' This means that the builders are still making the town. The town is not done yet, but the builders are currently working on it.