Race is a concept which classifies human beings according to broad physical characteristics such as skin color.
Various different classifications have been proposed for human races in the past. Evolutionary text books used to have a chapter on eugenics that discussed the 3 race breakdowns of Negroid (mostly Africans), Mongoloid (mostly Asian) and Caucasian (mostly Europeans) and which of the races were the furtherest along in evolution.
The concept of race is no longer considered scientifically valid, as there is very little genetic differences between the so-called races, and those differences are mostly superficial.
For example, all people have the same skin-coloring pigment (melanin), and what "color" one is depends mainly on how much melanin one has.
Whether or not it is scientifically valid, race is still requested in the government census and many other forms.