Heating a chicken coop in the winter is an oft-discussed and controversial topic. Healthy adult birds should not need heat under normal conditions. Heating a coop can also cause humidity issues, which can lead to respiratory infections or frostbite.
In some situations, heat can be beneficial. Chicks need to be kept warm until they feather out. Already-sick birds may need supplemental heat. Perhaps the temps are going to plummet quickly into the negatives. There are a lot of reasons someone might choose to provide heat, and each person should weigh the pros and cons carefully before making that decision.
If you ARE going to use heat - use SAFE heat!
Heat lamps are not safe. They are the #1 cause of coop fires. Even when secured and mounted well away from your birds, feather dust will build up on a bulb and can ignite.
A very good, and safe, alternative is a Greenhouse Heater. These heaters are made for greenhouse use, and can stand being splashed with water and dirt. They use infrared to heat objects around them. If they get too warm, a sensor will trip and they will turn off.
Yes, they are a bit pricey. But yes - they are worth it for the safety of your birds.