http://www.culturediversity.org/what is race.htm
http://www.aaanet.org/stmts/racepp.htm
There is also plenty of stuff out there to counter these arguments. So have a hunt and post that to.
When you speak of somebody's race what do you mean exactly?
And how many race's are there exactly?
As I said - the classifications are a human invention.
But, I usually see 5 - Caucasoid, Congoid, Capoid, Mongoloid, and Australoid.
However if race was not hard to define I doubt you see the amount of debate we see in the scientific community.
Race is a biological concept. The classifications - not so much. Classification systems aren't natural properties of the world, but human inventions, which are defined by human beings, for the purpose of making information easier for other humans to access and pass on.
Race is not hard to define, although definitions may change so as to not quite match what you see; in this case a wise man must accept the new scientific classifications. It doesn't take away the old guidelines. For example, people used to think dolphins were fish, and some people are still irate that we can't treat them as such, now that more advanced methods of classifying animals have put them in technically the same group as we people.