The other's answers are partially right, but take the common social constructivist view too far. Race/creed/caste/religion/clan/ethnicity are all slippery terms that are not easily defined. In a similar manner it is very hard to define what a species is (The common idea that a species is defined by when organisms can reproduce is violated for many species and meaningless for asexual organisms). A race or population from a biological perspective is a grouping of organisms that tend to breed more among themselves than with other groups. Subsequently they tend to have distinctive features. Now of course our common definitions of race have a large social influence that defies the logic of a group that tend to breed more with itself than other groups. For example the U.S.'s idea of hypodescent that dictates that someone who is 1/16 of black african descent is African American is kind of silly (on the other hand if these African Americans tend to breed mainly with other African Americans then perhaps it is not so far fetched).
Just because something can not easily be defined and overlaps does not mean it does not exist. Males are on average taller than females, but many females are taller than many males. This doesn't mean that it is a social construction to say meaner bigger then women.
To answer your original question, with these caveats acknowledged, genes are distinctive among different human populations (populations which follow human conceptions of race pretty well). Many of these genes are neutral and depend upon similar techniques to those used in forensics to get a DNA fingerprint. Interestingly, some of these gens are functional, they change the way our bodies and minds work.
For example, the sickle cell gene which helps protect against malaria is at much higher frequency in populations which must deal with malaria.
A genetic polymorphism, the dopamine receptor D4 gene varies radically among populations. This is a gene that has been associated with behavioral differences among people including ADHD. It is thought by some that different social background may select for different frequencies of the DRD4 gene. The frequency of ADHD associated genese in a population probably also impacts how that culture evolves.