Three Gunas

If Prakriti is the fabric of the universe, then the three gunas, Sattva, Rajas, and Tamas, would be the threads that make up the fabric. And like a fabric, the gunas are not seen as individual threads standing alone. They are always woven together to form a whole.
Sattva is that which is light, agreeable and pleasant, resulting in wisdom and illumination. Rajas is exciting, restless, and painful, serving the purpose of activity. Tamas is inertia; heavy, dark and sluggish, characterized by dullness and delusion, and serving the purpose of restraint.
The three gunas can be compared to the particles that make up the atom; that is, protons, neutrons, and electrons, having opposite electrical charges and in constant motion. But Sattva, Rajas, and Tamas are more subtle forms of matter, having their application in the development of character and personality, the transformation of consciousness, and the understanding of others.
Having defined Sattva, Rajas, and Tamas, Prakriti and Purusa can now be defined in terms of the gunas. Prakriti and its products are that which is constituted by the three gunas. Purusa is not constituted by the gunas. In these definitions we find the root of dualism. Purusa is the subject, Prakriti the object. The two are eternally distinct.
The primordial Prakriti consists of the three gunas in a state of equilibrium. In this state, Prakriti is unmanifested. The manifested universe consists of an infinite number of transformations of the gunas into various combinations. In these transformations, one guna predominates, and the other two are present in lesser degrees.
Take for example, the principle of Buddhi, the first evolute of Prakriti. The products of Buddhi in which Sattva predominates are said to be virtue, knowledge, dispassion, and power. The opposites (vice, ignorance, passion, and weakness) are produced by the "adjacent tincture" or influence of Rajas and Tamas.
We find other examples using the principal of Ahamkara (the I-maker), which has as its products the eleven Indriyas (or Instruments) and the five Tan-Matras (sound, color, form, flavor, and smell). Of these, the Indriyas of Action and Cognition are products of Rajas. In the Tan-Matras, Tamas predominates. The Sattvic eleventh is Manas or mind.
In the Bhagavad-Gita, the gunas are described as follows:

14.11 When Sattva predominates, the light of wisdom shines through every gate of the body.

14.12 When Rajas predominates, a person runs about pursuing selfish and greedy ends, driven by restlessness and desire.

14.13 When Tamas is dominant a person lives in darkness - slothful, confused, and easily infatuated.

The Bhagavad-Gita contains a number of examples that show how guna theory is applied. The automobile serves as a more modern example. An automobile can exhibit the qualities of Sattva, Rajas, and Tamas both in manufacture and ownership. A Sattvic car is well-designed and built, with adequate power, pleasant appearance, and good fuel economy. It is clean, well maintained and reliable. A Tamasic car is poorly designed and sloppily built. It may be dirty, dented, and cluttered with junk. Since it is not maintained properly, it breaks down at the most inopportune time. An automobile exhibiting the qualities of Rajas is loaded with horsepower, sleek, fast, and flashy, with every amenity. It seems to shout: "I am rich, sexy, and powerful. Economy is of no concern to me. Move over while I pass you."
Samkhya maintains that the gunas are the form of Prakriti, and not mere properties. It may seem fantastic to think that all of the manifested universe could be derived from combinations of just these three substances, but it is no more fantastic than the known truth that all matter is composed of protons, neutrons, and electrons. The skeptic may think of the gunas as a theory or model, but the reality of Sattva, Rajas, and Tamas cannot be absolutely denied, because their effects are observed everywhere.