The races of humanity constituted types of Homo sapiens. Classifications have to be based on something. For instance, the primary biological division classification known as Kingdom is based on observed differences between plants and animals. The classifications of both Homo and Sapiens are based on measurements of brain capacity and tooth size. Take a wild guess.
Skin color is the primary unit upon which type has been based.
It, in turn, refers to the phenotype, i.e. the physical appearance of the individual. More importantly it is a difference between variation and evolution. Evolutionary changes are irreversible. Variation, on the other hand, is much more malleable. Evolutionary changes usually can be reasonably well-fitted into a category, whereas variation does not fit very well at all.