It is the soul that gives life to the physical body, as stated in Tanya, and is the source of animalistic desires as well as innate Jewish characteristics such as kindness and compassion.
While the animal soul is still attached to worldly, physical pleasures, it is synonymous with the yetzer hara, the evil inclination.
Through persistent, joyous exposure to Torah study and observance of mitzvot, the nefesh habehamit can be trained to desire spiritual and divine pleasures.
In its refined state, it is often termed the nefesh hachiyunit, the life-giving soul, because its primary function then is to give life and energy to the body in order to study Torah and perform the mitzvot.
[6] The Tanya teaches that the nefesh habehamit is primarily manifest in the left ventricle of heart, and that from there, it spreads throughout the body via the blood.