Question: What is the animal soul? What is spirit?
Answer: The animal soul is the life-giving force that animates our bodies as well as the body of any other animal.
These are desires in which there is a small fraction of the Light that animates them. However, they need no screen, no restriction, no correspondence between the desire and the Light. Indeed, these desires are so microscopic that the Light can be in them in a very weak illumination. That is why matter exists.
The spirit is the same animal animating soul that exists in all the still, vegetative, and animate elements of creation. That is, it is still the desire and still the Light.
All people have desire, and the Light that supports this desire nourishes it a little and exists in it even contrary to the first restriction (Tzimtzum Aleph). This is the animal soul or spirit.
From the Daily Kabbalah Lesson 8/25/13, Question and Answers with Dr. Laitman