What are the biological differences between human races?

Race, as such, is a social construct. Genetic differences exist between races, but they are trivial in biological terms, because during the evolutionary period, all humans have a very recent common ancestry.

But while race is a social construct, many racial changes are determined by geography and common ancestry, and populations have traits introduced or enhanced by evolution in response to different environments.

For example, in some human populations there has actually been interbreeding with Neanderthals. There is a higher frequency of certain genes in populations that correlate roughly with certain races (eg French, Huns, and Chinese).