Infrared (IR) radiation is electromagnetic radiation that has bigger wavelength than visible light, but less than microwaves. The name means "below red" (from the Latin “infra”- "below"), as red is the color of visible light with the longest wavelength.
An influence of infrared waves is invisible for the naked eye. It is harmless as everything radiates infrared waves, in more or less intense ways. For example, sun radiates infrared waves.
In an IR sauna infrared rays hit your skin, they transfer heat energy to it. Your skin temperature goes up and you react by starting to sweat. But the distinctive peculiarity of IR is that they provide warmth from inside.

What is the effect of infrared radiation?
The both kinds of saunas have their health benefits. They include improved blood circulation and hence a stronger cardiovascular system, a cleansing of the body, and finally deep relaxation and refreshing. Although some people may be quite sceptic about many claimed benefits of sauna, but the fact is that many people enjoy sauna and feel better after using it.
Locally administered infrared is used as treatment for some skin diseases and tissue traumas. The most successful use of infrared rays is concerning joint pain and stiffness of arthritis patients. It eases pain and soothe. In such cases using infrared radiation has several advantages as it has effect of heating the skin and muscle tissue from the inside.
Although infrared rays are the same kind of electromagnetic waves as light, microwave, UV or x-rays, you do not have to fear. Remember that the heat we feel from the sun is an effect of infrared radiation and we feel ok with that as well.

How do IR saunas work?
In the traditional Finnish sauna heating is provided by pouring water on the heated stones. It takes a large amount of stones and a long time since the stones must first heat up. Throwing water on the stones in a traditional sauna brings up the moisture in the room. And you feel hotter at once since steam carries energy better than air.
In an IR sauna an infrared radiator sends energy directly to the skin of the bather that has an effect of heat waves. This energy penetrates the skin several millimeters (up to some centimeters), and reacts with the skin's tissues, increasing its temperature and quickly causing sweating.
Heat from an IR radiator behaves much like light from a lightbulb - it will only heat what is directly in the way of its rays. There should be infrared radiators at least in front of you and behind you in an infrared sauna. If there are not enough radiators in sauna then some parts of your body will not be visible to the rays and will not experience the heat properly.
IR radiator does not need extra time to heat up, it is usually ready to use as soon as you switch it on, whereas a traditional sauna takes about an hour to heat up.
If you’ve decided to try IR sauna nobody can promise you a hot sauna but IR radiator will help you to feel better and relaxed thorough inner warmth. Remember the most suitable case for curing by IR radiator is arthritis.

Traditional saunas and infrared heaters don't really compete in the same category. Traditional sauna is more than merely bathing or health benefits it’s a whole culture.