Darkeyes
Darkeyes are a caste of people from Alethkar on Roshar that are characterized by their dark eyes.[1]
In Alethkar, they are limited in maximum attainable status due to their being the socially inferior race, although they seem to make up the majority of the population of the nation. Due to the fact that darkeyes are never nobility in Alethkar, it has also become virtually synonymous with peasantry.
All darkeyes are not equal, however. There are ten different levels, or 'nahns', of darkeyes. For example, Kaladin's family is considered to be 'of the second nahn'.
A similar system is used to classify lighteyes. Instead of 'nahn', however, they use 'dahn'. A darkeyes of the first nahn can actually be more influential and wealthy than a lighteyes of the tenth dahn, although technically, the lighteyes is of a higher ranking. A darkeyes of one of the higher nahns is permitted to wed a lighteyes of a lower dahn.
It is strongly implied that it is a Vorin tradition. As such, countries that do not follow Vorinism generally don't have eye colour based classes. Sigzil points out numerous other systems of governance.[2] Therefore, it stands to reason that it is not a universally recognized racial classification system.