If you are keeping your chickens in order to raise them, and not just for egg-laying purposes, then you will need to be aware of how to keep the temperature in the right range. Baby chicks need it to be quite hot when they're first hatched. It is for this reason that mother hens will sit with them in the nest for the first 48 hours, keeping her wings spread over them for warmth. In the wild, hens will often keep up this kind of behavior for a few weeks, until the chicks are feathered enough to withstand normal temperatures.
If you're raising chicks in a special environment, make sure that you keep the temperature hot enough for them. The easiest way to do this in a controlled situation is by the use of heat lamps where the chicks are living. For the first week, chicks need to be kept in an environment that is around 95 degrees Fahrenheit, or 35 degrees Celsius After that, lower the temperature by 5 degrees Fahrenheit (roughly 3 degrees Celsius) for each week, until the chicks have reached an age of around 6 weeks old. At this point in their life, baby chicks should have gained enough feathers to go outside in the normal chicken area. Unless, of course, you live somewhere exceptionally cold.
Fully Matured Chickens
Once they have fully matured, chickens usually prefer temperatures that are neither extremely hot nor extremely cold. You can ensure that your chickens stay comfortable year-round by building them a well-ventilated coop with solid walls and plenty of air circulation. If you live in a particularly cold part of the world, you will probably want to consider adding some sort of insulation into your walls to help keep your chickens warm during winter. If poultry get too cold, their egg-laying rate will go down drastically, so it's in your best interest as well as your birds' to keep them nice and comfortable.
During warmer seasons, or if you live somewhere particularly warm or humid, make sure your birds do not get too hot. Chickens are actually better at withstanding cold than heat, so this is important. The easiest way to keep the temperature comfortable when it's hot is to adequately ventilate your coop. Make sure there is a nice fresh breeze, but that it's not causing a strong draft. You can accomplish this by building windows into your coop and by opening or closing them until the amount of air going through is just enough to keep your birds at a comfortable temperature. A coop that is too hot and humid can cause disease amongst your chickens.
In addition to just structurally reinforcing your coop, it is possible to purchase artificial means of keeping your coop at a nice temperature. Just like for our own homes, there are plenty of heating units and air conditioning units that you can buy for your chicken coop. As with any animals, if you go this route you will want to make sure that you cover up the wires of the system so the chickens cannot get to it. This is especially true since the heating system's wires will be exposed to the elements, so that any damage to them may lead to dangerous or downright disastrous circumstances.