This summarizes what Krishna teaches in Bhagavad-Gita, chapter 18, texts 20 through 39. It is an except from my book (available in May 2013), “An Interview With Krishna.”
Sattva – Clarity
Clarity sees the unity in all things. Clear people don’t differentiate “desirable” from “undesirable.” Thus their actions are motivated not by desire but by duty. Their intellect is set on clarifying what is their duty and what is not. Their willpower is fixed on not allowing themselves to be deviated from their duty. Although this all seems to disciplined and strict, clear people are the happiest of all because by stilling their external self they come into contact with their blissful inner self.
Rajas – Passion
Passion sees things as being essentially different. Therefore passionate people find some things “desirable” and others “undesirable.” Based on that, they act with the motive to obtain the desirable and avoid the undesirable; they don’t act out of duty. Their intellect doesn’t care to distinguish what is dutiful from what is not; it is obsessed instead with differentiating the desirable from the undesirable. They apply their willpower to achieve the things they desire. Although this seems like fun, it winds up exhausting and lands them in conflict and misery.
Tamas – Darkness
Darkness doesn’t really care about anything, so it produces haphazard and aimless actions. The darkened criticize others but do nothing themselves, except procrastinate. Their intellect doesn’t distinguish between anything – everything is the same to them: duty and non-duty, desirable and undesirable. Therefore they have no ambition or willpower for anything other than fantasy and dreams. Happiness never really exists for them, outside of dreams.