The three most common types of central humidifiers are:
Drum type, where a drum holds water which naturally evaporates into the airflow through your ducts.
Flow-through, where a water line drips into a catch pan inside your ducts, as the air blows through the “rain” that this type creates.
Active evaporation, also known as a misting or steam humidifier, which rapidly heats small amounts of water to keep a constant flow of steam entering your airflow.