In philosophy, dualism refers to the idea that there are two types of stuff in the universe. One type is the matter and energy that traditional physics deals with. The other is mind, or soul, or spirit. There has been much debate as to whether dualism actually makes logical sense, and if it does, if it has more explanatory power than monism.
Dualism comes in three basic types:
Dualistic interactionism; The mind (or soul or spirit) and the body (atoms and molecules and meat) interact with each other. This version is most popular, and is the default assumption for much of the human race.
Parallelism; The mind and body exist alongside one another, but do not effect or interact with the other.
Epiphenomenalism; The mind is just a by-product of the material world. (Like the sound of a babbling brook--the sound is caused by the water, but does not effect the water, and the sound of one moment does not affect the sound of the next)