There are 3 basic types of humidifiers for CPAP machines.
Heat and moisture exchange (HME) humidification
Passover Humidification: The passover humidification is unheated and just passes the air over the surface of some water. The water will evaporate into the moving air thereby increasing the humidity. The amount of humidification accomplished is minimal and not controllable but it does not require any electricity nor does it require maintaining the cleanliness of the heater surface and heat transfer plate. It can increase patient comfort by reducing the dryness of the compressed air. In general, a passive humidifier is a separate unit and does not have a power source.
Heated Humidification: Heated water chamber that can increase patient comfort by eliminating the dryness of the compressed air. Heated humidification is basically the same as passover humidification with the addition of a heat source to increase and control the evaporation of water into the air going to the victim, errrr scratch that, patient. The simpler heated humidifiers merely have a number of manually adjusted settings that make the humidity increase more or less. The more sophisticated heated humidifiers have control circuits that control the humidity going to the mask. The advantages are the ability to add more humidity to the air and the ability to control the amount added, The disadvantages are more electricity required and more maintenance.
HME: Heat and moisture exchange (HME) humidification picks up moisture from the humid exhaled air and gives it up to the less humid air being inhaled. It is done by a porous media that has been treated with something like silica gel which picks up moisture from the air. HME humidification is simple, inexpensive and can be made relatively small (nice for travel-especially in the outback). It is not controllable or adjustable and must be replaced on a weekly basis. The initial investment is small but over a year ti becomes significant enough to be a consideration.
There are varying opinions on what to use for CPAP humidifier water.