When the city reach few hundred of citizens, you should allocate some regions which will function under common name, like residential districts and territories focused on specific industry direction.

Districts allow you to diversify city resources and increase tax income. They give you greater control over your citizens, by allowing you to make district regulations. They also show you general information about a specific territory.

At the beginning you can't create advanced districts with mining or oil industry, but on the map you will definitely find forests and fertile grounds that will help you increase your income.