A sauna is a room heated to a temperature higher than normal. People go into saunas to sweat. Normal sauna temperatures range between 60-100° Celsius; some like it hot and some slightly hotter, but it is just a question of personal preference. Air moisture inside saunas varies depending on how much water has been thrown on the rocks and how hot the stove is. If the air is very dry, it becomes uncomfortable to breathe.
Sauna stoves can be heated with wood or electricity1. Stoves that use wood normally have a chimney, but the most traditional version doesn't and therefore it is called a smoke sauna.
All smoke sauna stoves (and some of the stoves with chimneys) are called 'once-heated' stoves. In this system, the stove has a very large rock mass (400-500kg) that preserves enough heat for several hours of serious bathing. Heating takes anywhere from three to six hours depending on the size of the stove and the sauna.
'Continuously-heated' stoves are far more common nowadays, since they are much easier and faster to heat. A fire is kept burning, or electrics are kept running, during the bathing and stove has only 20-40kg of stones.
Benches (Lauteet in Finnish) are made of untreated wood - metal would feel too hot on naked skin and plastic seats would melt. Normally benches are on one or two walls and rise stepwise from the door. Normally there are three steps and you sit on the third and keep your feet on the second2.
What To Do in a Sauna
Sit down or better yet, lay down if there is enough room. Let your mind wonder and relax. Throw water on the rocks and enjoy the feeling of steam hitting you. If you are curious enough, try beating yourself with a sauna whisk.
In saunas there are really no 'have-tos'; you don't have to stay in for a fixed period, a sauna doesn't have to be such-and-such a temperature, etc. Just listen to your own body and do what feels good. If you start feeling uncomfortable, get out, cool down3 and go in again or quit altogether.
Swimming as Part of Having a Sauna
Finns prefer their saunas by the lake, sea, pond or river. Nothing refreshes you more than a plunge in to cool water and due to its climate, water in Finland is quite often cool. During winter months swimming means a little bit more effort since you might need to make a hole in the ice. Of course not all saunas are next to water4, but plunge pools with cold water or even normal pools are good substitutes. Some people like sitting in a hot tub as part of their sauna rituals.
Signs That You Are Not in a Real Sauna
Any of the following is alone enough to prove that the sauna you have entered is not a true Finnish one:
The floor is covered with wall-to-wall carpet.
There's no floor drain.
There are no stones in the electric stove.
There is a clear minute-by-minute timetable about proper usage of sauna5.
Benches are made from metal, plastic or painted wood.
Thermostat won't go over 40 or 50° Celsius.
There is an American-style warning label nailed to the door.
1 Actually, saunas can also be heated with oil, peat or anything else that burns or produces heat.
2 It is not shameful to sit on the second one and keep your feet on the first one, if it is too hot on the top bench.
3 The fastest way to cool down is to have a roll in the snow.
4 Especially true outside Finland.
5 Quite common in Germany.