"Early examples of factory-like production, where a number of individuals work cooperatively to produce goods for sale or trade, can be found as far back as ancient Greece and Rome. Modern factories, however, evolved from the concept of specialized labor, where each worker performed a single step in the overall production of an item. This specialization allows factories to increase the speed and efficiency of the manufacturing process, far surpassing earlier means of production. The development of the factory system as a means of production played a key role in the Industrial Revolution."
"Factories increase a city's production at the cost of adding significant unhealthiness. Factories are required to build Coal, Hydro, or Nuclear Plants, which add additional production with the proper resources."
In the city screen press the following icon to start Factory construction: