Hatred, contempt, pity, and patronization are the words I remember Howard Zinn using to describe racism in one of his books. I disagree with pity, and I'm questioning patronization, but hatred and contempt are definitely correct. I haven't discriminated against any race, but I remember hating the entirety of the underclassmen student body when I was in middle school. Due to the way I saw them behave during the gym period, I felt (didn't think; I'll get to that later) that they were the dumbest little monkeys on the face of the planet. Even seeing one of them was enough to annoy me, and if one of them approached me, I would treat them with those exact two words that I agreed with: hatred and contempt. To be more specific, I treated them with hatred as a result of contempt; the feeling of thinking they're naturally inferior and that they deserved scorn. Of course, I didn't think this but felt this since I was judging the entirety of the lower grades based on what I felt when seeing a fraction of its boys, and not even the fraction I judged were as bad as I thought since I was just going by what I saw on the surface. After some thinking, I came to the conclusion that most underclassmen weren't that bad, and that the fraction I judged were, at the very most, just annoying and not inferior. I'd say that me judging this entire group of people with harsh, unfair feelings is the closest I've gotten towards racism.
Anyway, what I can gather from this experience is that racism is born out of what people feel, not think. Another important factor I've noticed from other people is the lack of nuanced reasoning when they are thinking about it. I remember one of my peers telling me that "Black people have never treated me with respect." He was going by his own personal experience with the small fraction of the entire race that he had met. It was also likely that he had met blacks who treated him with respect, but he just didn't consider them in his reasoning over the jerks he came across. Nuanced reasoning can go farther than just "Not all of them are bad." I remember one poster in this thread talking about his brother's jealously with affirmative action. For the sake of argument, let's suppose that we're against affirmative action. Even if we are, is that any reason to hate the people who receive the benefits? It's difficult to make it in this world, and they're just trying to get by on whatever opportunities they can get like we are. The proper course of action would be to go against the politicians who put affirmative action in place, not hate the people who are receiving the benefits.
Action based on feelings, lack of nuanced reasoning when using one's head, and I'm pretty sure ignorance also plays a role. I remember one KKK member shouting over a megaphone that the only reason why the blacks were free and had equal rights was because white people stood up for them. It's worth noting that white abolitionists during the pre-civil war era (with the exception of people like John Brown) were barely anti-slavery, and their support for the cause was often half-butted (in more polite terms). It was free blacks at the time such as Frederick Douglass who played important roles in the abolitionist movement. During the civil war, black regiments helped out with a lot of the fighting on the union side. Blacks who were enslaved would frequently disobey plantation owners by working slowly, breaking tools, and occasionally, try to escape and even attempt to form slave revolts. Needless to say, they were far from being subservient.
I hear people argue whether racism is natural or learned; I believe that it can either be learned, or triggered by personal experience and permitted to stay by lack of sufficient thought and not enough good information.