Since as far as I remember, I've always heard that colleges or universities in the US have a sort of special preference for admitting people that are considered "minorities". I would like to know if this is true, and if so, what it entails.
It's sometimes but not always true, and at schools where it is true, the definition of the preferred groups varies. For example, in 1996 Californians passed Proposition 209, which, among other things, requires public schools to have admissions policies that are blind with respect to race, sex, and ethnicity. So, for example, UC Berkeley is not supposed to give preferences for admission to an African-American applicant, but Stanford can (and probably does). It would be up to Stanford to define their preferences.
Many private schools have some admissions policies that, considered by themselves, would tend to exclude disadvantaged students. For example, MIT has need-blind admissions, but RPI isn't need-blind and doesn't have a policy of meeting full demonstrated need. This would tend to reduce access to RPI for students who come from working-class families. Many private schools have a practice called "legacy preferences," which means that they are more likely to admit the children of alumni; for example, George W. Bush would have benefited from such a policy when he applied to Yale. One of the original purposes of legacy preferences was to exclude Jews. Being Asian is probably a disadvantage in college admissions. A 2004 study by Espenshade et al. puts the admissions penalty for Asians at the equivalent of about -50 on a the old 1600-point SAT scale.
At places like California public universities where there is no longer affirmative action, politicians and administrators have invented a number of ways of trying to preserve "diversity." For example, a certain number of spots are reserved for students who rank high in their high school's graduating class, even if the school's academic standards and offerings are weak. Admissions officers are said to look for whether the student has taken the most challenging curriculum offered at their high school, so, e.g., a student at an elite public high school that has an IB program could be at a disadvantage if s/he didn't do IB.