Traditional saunas can be indoor or outdoor. They use electric or wood-burning heaters to heat the air inside the sauna up to temperatures of 190 - 195°F.
By sitting inside the heated space, your body temperature also rises, causing you to sweat. Traditional saunas offer higher humidity levels, especially when used with water on the rocks to make steam.
Infrared saunas are typically indoor only, and they use penetrating infrared light to heat the body directly, rather than the sauna space. Infrared heaters use carbon heaters to generate heat up to 120 - 140°F, raising your body temperature directly to cause sweating.