Löyly is a special feature of Finnish sauna, meaning:
1) Steam or vapour created by throwing water on the stones of the kiuas
2) The heat, humidity and temperature in the sauna in general
In a modern Finnish home, throwing water on electric equipment is done every week. That is, throwing water on the electric kiuas (stove). The stones are so hot that the water evaporates almost immetiadely with a hissing sound. If the sauna is sufficiently hot, that is, over 60 C, the steam disappears into the air almost immediately. No clouds are formed.
The air gets so humid that most of the "sweat" is condensated löyly: sweating is NOT the point of sauna. The cohesive force of water is the reason why the steam will condensate on the persons rather than on the walls or benches. The condensation (opposite of boiling) gives out heat to the person sitting in the sauna, which gives the impression that "temperature has risen". Also, the humidity increases the thermal conductivity of air and skin,so an increased rate of heat transfer is experienced as a temperature change. Actually, temperature drops few degrees, because the steam loses thermal energy when it condensates.
Other peoples have also sauna-like baths, but only Finns and Native Americans are known to increase the humidity in this way. The heat feels a lot hotter, when the air is more humid, because more heat is conducted to the skin. It's easy to moderate the heat with löyly.
A good löyly is an fast, invisible hot rush of heat, which saturates to a sensation of pleasant... err... sauna feeling. You relax and feel good. Too hot löyly hurts the tips of the ears. Squeezing the tips of your ears between your fingers eliminates this danger. It's important to understand that it burns when the skin is heated, not simply when the surrounding air is hot. A hot löyly will heat most the areas with little mass and a lot of skin area, such as ears, but not the face. The large areas, such as the back, don't heat up as quickly.
Again, it must be emphasized that löyly is fundamentally different from a hot weather. The question "how is löyly different from hot weather" deserves the answer "mu". The question assumes equal radiation, humidity and temperature. For example, hot weather cannot have the power to burn the skin in ten seconds.
No saline nor too dirty water can be used for this. Salt will make the iron corrode quickly. Lake water can be used, but it may contain so much humus that the löyly smells strange or bad. Rain water is better, but see that no frogs or spiders get thrown on the hot kiuas. In the summer cottage with no water pipes it's a good idea to put a large barrel under the drainpipe and cover it with a piece of cloth. This way no solids can contaminate the water. Bringing all the needed water from home is impractical.
The word löyly traces back to Proto-Finno-Ugric, which was spoken 6000 years ago, and is reflected in Hungarian as lélek "soul". A single burst of löyly is often called löylyn henkäys "a breath of löyly". Pre-modern Finns (which, in this case, counts as up to 19th century) placed löyly in the hierarchy of elements, along with e.g. water, earth, death (kalma) and metal. The elements could cause poisonings (vihat), which would be exhibited as illnesses, or infections in open wounds; thus, chanting of spells and magical rituals were necessary if one would enter the löyly with a broken skin. Older saunas didn't have wooden floors, and were dirty, although the sauna was for certain the cleanest place in a pre-modern household. Indeed, condensed löyly was one of the few pre-modern sources of sterilized water. Modern saunas have tile or wooden floors and as such, are clean.