What is race?

I have talked about race a lot in my life. I have thought about it even more. (The whole growing-up-biracial thing tend to do that.) I realized today, though, that I'm still not sure exactly what that is. My urban education class just finished reading Jay MacLeod's Ain't No Makin' It, which looks at a small sample of "project kids" and their experiences over twenty years. (Quite an interesting read.) Some of them (mostly white) have checked out of the system, rejecting school because they don't think that it will make any difference in their circumstances. Another group (mostly black) are determined to play the game and make something of themselves by graduating from high school. The results are varied (and the sample size is small), but to me the most startling aspect was how much worse off the black students who checked out of the game ended up than their mostly white friends- although all were poor, addicted to drugs, and had frequent run-ins with the police. It seems as though race matters more than attitude, even among people in the same socioeconomic class. Now I am wondering: what do I do with this? The truth is, working hard will not necessarily fix your problems when the deck is so stacked against you, but not working at all makes things even worse.
Back to my original question: what is race? I have no idea. To some extent its a social construct, but it has a physical (biological) aspect as well. The genetics of race are still hazy (are you what you have the most genes of? How do you define someone with genes from multiple races, which is most of us these days?) Race affects how people view you and how you view others. It can be a way to define who is like you and who is not. It gets tangled up with ethnicity and appearance. Race can mean skin color or eye shape. It can mean who you were raised by. Although race has played such a major role in how I think about the world, I have trouble coming up with a concise definition of just what it means. Thoughts?