According to the English pages of Wikipedia:
A sauna is a small room or house designed as a place to experience dry or wet heat sessions, or an establishment with one or more of these and auxiliary facilities. These facilities derive from the Finnish sauna.
The word sauna is an ancient Finnish word referring to the traditional Finnish bath as well as to the bathhouse itself. The oldest known saunas were pits dug in a slope in the ground and primarily used as dwellings in winter. The sauna featured a fireplace where stones were heated to a high temperature. Water was thrown over the hot stones to produce steam and to give a sensation of increased heat. This would raise the apparent temperature so high that people could take off their clothes.
Eventually the sauna evolved to use a metal woodstove with a chimney. Steam vapor was created by splashing water on the heated rocks.The steam and high heat caused bathers to perspire.
The Finns also used the sauna as a place to cleanse the mind, rejuvenate and refresh the spirit, and prepare the dead for burial. The sauna was (and still is) an important part of daily life, and families bathed together in the home sauna. Indeed, the sauna was originally meant to be a place of mystical nature where gender/sex differences did not exist. Because the sauna was often the cleanest structure and had water readily available, Finnish women also gave birth in the sauna.
The lighting in a sauna is shady, and some Finns prefer to sit in the sauna in silence, relaxing. The temperature is usually between 80°C (176°F) and 110°C (230°F). Sometimes people make a 'vihta'; they tie together small fresh birch branches (with leaves on) and swat themselves and their fellow sauna bathers with it. One can even buy vihtas from a shop and store them into the freezer for later (winter) use. Using a vihta improves blood circulation, and its birch odour is considered pleasing.
Here is a sauna located at a summer cabin, which we Finns often use during all the weekends in the summer and even the holidays are spent there.
and this is a typical sauna in an apartment or a house:
Lately I have seen lots of systems they call saunas. To a Finn these are not saunas but some kind of equipment which has one or two similarities of a real Sauna.
1. Infrared Sauna, Burn Calories While Doing Nothing
Healthmate's sauna uses infrared technology to make your sauna-going experience all the more pleasurable. Rather than having to heat the air and have steam do all the work, the infrared heats the body directly. The company claims that within 30 minutes users can lose up to 1,000 calories. Additionally, all that warming warmth has other health benefits, including an increased blood flow that may help with things like arthritis and sprains. (Gizmodo)
This works on lower temperatures, has no steam, no water... It's just a warm box looking like a sauna!
2. Solo Personal, Portable Sauna
This is essentially a tube-like tent that will get all hot and frothy so you can sweat away all of that negative karma you received from a hooker earlier in the day. The Solo personal sauna can operate for up to 12 hours at a time if you need to get rid of a lot of negative karma from a particularly dirty hooker. (Gizmodo)
This has actually only the warmth and that can't be even close to that of a sauna. It's more like a tanning bed without the effect of tanning. I think I would only get more negative karma lying in a tube like that.
3. Energy Cocoon Tub: Room for One More?
Install an entire spa in your bathroom with this Energy Cocoon Tub, giving you an invigorating combination of soothing bubbles, billowing steam and an infrared sauna all in one unit the size of a regular whirlpool tub. It contains three multi-colored lamps for a bit of that soothing chromatherapy, and of course, it has those Jacuzzi-like waterjets, too. Then when you're done, you can rinse off with its hand shower. (Gizmodo)
This has at least water and steam but does not remind and certainly not will feel like a sauna.
It's actually a sauna and shower combo, complete with luxuries like a digital remote, foot massager, aromatherapy, and music and phone ports. You'll be so busy in there you'll forget to do basic things such as wash your hair. (Gizmodo)
This has the basic elements of a sauna, but this is for one person only. Also saunas are places where you should forget all technigal gadgets and thus the music and phone ports should definitely not be included. The foot massager and aromatherapy can be accepted if we take a liberal approach to sauna culture. I just wonder why it has a remote. It is so small, that everything is within reach.
These are from the 70's! You can surely get swetty in them but I can only imagine how uncomfortable one must feel after using these (Boing Boing)
6.Ice Sauna Goes to 10 Below Zero, Causes Shrinkage
The Snow Room was developed by MNK—a company that has made a name for themselves developing saunas. While the idea does seem strange, it appears that hot/cold therapy is common in northern European countries. So the idea of sitting in what is essentially a meat locker for relaxation may not be so far fetched after all. (Gizmodo)
Hot/cold therapy mean that we go from the sauna to roll in the snow or to swim in a frozen lake. This could be an attachment of a real sauna in the summertime when the water in the lake is warm.
7. The Sauna Suit or the Personal Sauna
These are both designed for weight loss. The suit makes you sweat yourself thin. While it does reduce weight by DEHYDRATING you, the effects of the sauna suit are sure to be short lived. Operating under the same principle as the sauna suit, the personal sauna promises to help you shed unwanted weight again by sweating it off. Just set your temperature and timer then voila…a whole new you! Do you even think a 300-pound man can fit inside that contraption? (THE LaparoscopicBand CENTRE)
Like stated above, these make you just sweat and that's all!
The Wellness Skull houses a bath in its neck, a sauna in the head, and hot steam spouts at the eye sockets. There's no pretentious receptionist or wind-chime music to help you chill out—stepping into the skull will instantly take away the worries of contemporary society and fill you with thoughts of life, death, and the emptiness of our physical selves. It's like an instant dose of existential meditation. (io9)
The inside of the skull is not shown, but I think that no Finn would build a sauna like this.
A Russian inventor has built this mobile sauna. Converted from a military truck, Igor Chupin offers clients a session in the back of his GAZ-66 truck for 1000 roubles ($42.37) per hour. Chupin said it took him months to build the traveling banya offers the real Russian experience. It has a 400-litre (88-gallon) water tank on top to provide the steam. It is heated with a wood-fired stove.
This is the only one I would accept as a real sauna. Wood-fired stove, the steam,... You could just drive this to the nearest shore and cool yourself in the lake or the sea between the steam sessions.