The three gunas (characteristics) namely, Satva, Rajas, and Tamas- are the three fundamental energies of the mind. Psychological characteristics of the person are dependent on the relative dominance of the three gunas.
Though all individuals have a combination of all of the three, the predominant guna determines an individual's psychological characteristics. When in equilibrium, the three gunas preserve the mind (and indirectly the body), maintaining it in a healthy state. Any disturbance in this equilibrium results in various types of mental disorders.
Satva is characterized by the purity of mind as lightness, consciousness, pleasure, and clarity. It activates the senses and is responsible for the perception of knowledge.
Rajas are characterized by the motion and stimulation that leads to desires, wishes, and ambitions.
Tamas is characterized by heaviness and resistance that produces disturbances in the process of perception and activities of the mind. Over saturation of tamas guna leads to delusion, false knowledge, laziness, apathy, sleep, and drowsiness.