"Idealists" think that the mind is all that exists and that the outside world is actually made up by the mind. Physicalists think that everything can be expressed by what is physical. "Neutral monists" believe that everything can be either mental or physical depending how you see it. For example, a red spot on a wall is physical, because it is an actual thing depending on the physical wall, but it is mental because our brain responds to the colour. The most common monisms in the 20th and 21st centuries have all been different kinds of physicalism, including behaviorism.