Chimneys often have a covering on them known as a Cowl, they are often hood shaped and increase the draft that goes down the chimney, this prevents back flow.
They can be made from different materials but the most common is Galvanized Iron, this is extremely effective at stopping the release smoke from blowing back into the house (building).
They have been manufactured and used for centuries and resemble garments worn by monks (this is were they got their name).
If you have an open fire in your home, when lit it works by the smoke rising up through a flue, it then continues to rise up to a chimney pot located on the roof of the building.
The smoke is then dispersed into the air where it cannot cause you harm, if chimney cowls were not used the smoke can be pushed back down into the room, this can cause harm to you (lungs) putting you at risk, it can also make your furnishing dirty.
Smoke poses a very serious health risk and should always be sent out of a room.
In the past before the Clean Air Legislation was put in place, Raw Coal was used for fires and carried a significant amount of smoke, the Cowl was pretty much essential back then for your own safety.
Now smokeless fuel can be used the Cowl is not quite as important but does still offer benefits.
An alternative today are electric fires, these can be made to look like a natural fire but without the risks and smoke.