For a fairly lucid description of affirmative action, as well as the controversies surrounding it, click on this link to the Stanford Encyclopedia of Philosophy. And for a more in-depth collection of essays and articles about affirmative action, click on this link to "The Affirmative Action and Diversity Project" from UCSB.
There is a lot of confusion about what affirmative action is and who it benefits. And the truth is, it's not perfect--there have been problems in its execution and interpretation. People generally think of affirmative action as a program for "minorities" Yet one of the main beneficiaries of affirmative action policies have been white women. Others point out that Asian Americans have profited over affirmative action programs that were really designed to help enfranchise African American, Latino, and American Indian groups--ones who had faced more systematic racism than Asians in America. Although I would parry by showing that Asian Americans also face racial discrimination, although perhaps it does not look the same as the types of discrimination faced by others, and that their "success" should be qualified since I don't see many Asian Americans in positions of power--in other words, I'm stil banging my head on that glass ceiling. Still others (and this is, I think, what people think about when they think about detractors of affirmative action) believe that it is a system that unfairly promotes unqualified "minorities" over more qualified "whites." And one of the more contested areas is around college admissions.

The above cartoon really sums up Chapter 7 of critical race scholar Robert Chang--I've already promoted his work, and that of Scott Page in the post "Reverse Racism!" so I won't repeat myself here.
I'll just end with an observation: if, as many people believe, that affirmative action isn't needed because we are on an equal playing field--that women and people of color are not facing institutional discrimination or social disenfranchisement, why are we still having a national conversation about whether a white woman or a black man can be "presidential" material? Or perhaps even more prosaic, looking around at the heads of colleges and universities, looking at Congressional representatives and senators, looking at the CEOs of Fortune 500 companies and heads of Hollywood movie studios--do we really think that men of color and women of all races are being equitably represented in positions of power?
I said I would stay positive, and since this is the eve of the Chinese New Year, what I will end with is saying that I do, truly, appreciate affirmative action. My life is better because I have been able to live in a society that values diversity, and I am privileged to work in environments that value diversity and that work to end social disparities in gender and race (and class and sexual orientation for that matter). I appreciate affirmative action not because of what I have personally gained from it but because it has made my life richer by allowing me to hope for a more equitable world.