A Guna is a state of mind--an attitude. Attittudes are basically qualities or tones of vibration and are found in everything, especially in the human. There are three Gunas, or attitudes, from which all the more subtle attitudes are derived. The three Gunas bind your spirit to the body, keeps it here. A Guna is an earthly quality. The three Gunas are Sattva, Rajas and Tamas.
Tamas
Tamas or inertia, binds the spirit by laziness, ignorance and too much sleep. It is the trickster of Jeeva, which is female or soul. Tamas attaches you to ignorance by hiding knowledge from you. Tamas dominates by suppressing Rajas (activity) and Sattva (goodness). Ignorance, laziness, carelessness, and delusion arise when Tamas is predominant. If you die while Tamas rules, you are reborn again as a Taamasika person (someone in whom Tamas is ruling). If you do anything while Tamas is ruling in your life, the consequence will be ignorance. Negligence, delusion, and ignorance arise from Tamas. Taamasika persons, living in the lowest Guna, basically experience hell. They are repeatedly born as lower castes creatures of humankind.
A Taamasika person does not really do anything. They wake up late, eat and often, take alcohol - all day. This is a very generalized statement. They are lazy and ignorant. They are not interested in learning anything, people often have a hard time changing a Taamasika person's mind. They neglect their duties, do not care about other people much.
Rajas
Rajas or selfish activity is seen in people that are chasing materialistic or egocentric dreams. Rajas is born by desires and the results of work, for example: unnecessary nice material trinkets and ego decorations. Rajas binds you to action\work. Rajas rules by suppressing Tamas and Sattva. Greed, activity, restlessness, passion, and the doing of selfish works arise when Raja is ruling in your life. If you die while Rajas rules in your life, you are re-incarnated as a person in which Rajas rules. The consequence of Raajasika action (a selfish action) is pain. Raajasika persons are reborn in the mortal world, or earth.
We often meet people that work extremely hard, they cannot stop working. Most of the time they are rich by average standards (few people in the world own cars and brick houses and PCs), care too much about their cars or houses. They would do almost anything for money. They live selfish, greedy, restless and very active lives.
Sattva
Sattva or goodness, is calm and clear. If Sattva rules in a person, they always do the right thing. Sattva binds the spirit to the body with happiness and knowledge. Sattva rules by suppressing Rajas and Tamas. When goodness and wisdom are present in your whole being, flows out from your whole self - then Sattva is ruling. If you die while Sattva rules in your life, you go to heaven, the pure world where the knowers of the Supreme live. The consequence of Saatvika action (a good action) is pure. Knowledge comes from Sattva. Those who are established in Sattva go to heaven.
A Saatvika person is sometimes hard to identify because they are so humble, they do not make it known that Sattva rules them. The saints and Yesu were Saatvika people, but Yesu rose above Sattva. He conquered the Gunas.
Those who can conquer the Gunas, according to the Bhagavad Gita, get to abide in the eternal Happiness. When you realise that you can conquer the Gunas, you are free from birth, old age, disease and death.