There are a few different types of humidifiers out there. Here are the 3 main types:
Vaporizers. These literally just heat the water until it boils, and the steam evaporates into the air. They use a lot of electricity, which also creates a lot of heat (note: exactly the same amount of you have an electric resistor furnace). They can be a safety risk (because boiling water), and require frequent cleaning/descaling if you have hard water. IMHO, these also make the air feel more humid.
Evaporators. These are basically just a fan over a wet towel. Very cheap, very low energy. Often less effective. As they are effectively standing water, you have to be careful to keep them clean or the nasties will grow quickly. This is also the type that “whole home” humidifiers typically are.
Ultrasonic. These are very popular these days, and may randomly be sold as cool mist, warm mist (if they have a small heating element). Low energy options that vibrate the water into a fine mist, which then evaporates. But they will also vibrate everything else in your water into the air. These deposits, like calcium or limestone, can affect your breathing. They will also leave dust all over everything you own. Recommend using distilled water for these.