There are usually 2 kinds of evaporative diffusers, ones that use a fan and ones that use the existing air current. Both kinds of evaporative diffusers are pretty basic in how they operate.
With an evaporative fan diffuser, a fan blows air from the room through some sort of pad or filter that has essential oils dropped onto it. The air blowing through the pad causes the oils to evaporate more quickly than normal, and the air with the evaporated oil is blown into the room.
The other type of evaporative diffuser doesn’t require a fan to blow the oil around but instead uses the air current existing in the room to subtly diffuse around a small area.