There are a couple of key differences and these relate to the heat source and the humidity.
Heat Source:
A traditional sauna heats the air inside the sauna room, typically with electric or wood-burning stoves, creating high temperatures that indirectly warm your body.
An infrared sauna emits a type of heat that directly warms your body, similar to the way the sun's rays heat your skin.
Humidity:
A regular sauna has high humidity ranging from 10-40%, therefore it needs ventilation.
An infrared sauna does not produce steam or increase the humidity in the room - so it does not need any ventilation.