There are four basic types of essential oils diffusers.

Ultrasonic Diffusers

Ultrasonic diffusers (also called humidifying diffusers) create a fine mist that mixes with the air. What a humidifying diffuser does is use ultrasonic waves to vibrate a small disc or platform which is submerged in some liquid. Usually water is used. The vibrations occur at a very high rate, and when you add essential oils to the water, those oils are broken down into microscopic particles.

Nebulizer Diffuser

These are waterless diffusers that use pressurized air streams from specially designed jet nozzles. This causes the breakdown of essential oils into very tiny particles. 

Evaporative Diffuser

This type uses air flow to diffuse the oils. A fan is used to blow air across a filter, stone or pad upon which a few drops of essential oil is placed. 

Heat Diffusers

Like the aforementioned evaporative diffuser, this process causes essential oils to evaporate at a higher rate than other diffusing methods. However, heat diffusers are budget-friendly and totally silent.