The people of Cyrodiil are divided into two ethno-cultural groups: Nibeneans and Colovians. The phrase 'Imperial Race' is a bit of a misnomer. In The Elder Scrolls V: Skyrim, if they are not part of the Imperial Legion, they typically serve as diplomats, traders, stewards, and craftsmen - which is well-suited, since Imperials are renowned for their diplomatic skills and cosmopolitan ideology. Imperials also have slightly sharper faces than other races of men. The Imperial skin tone is not as dark as that of the Redguards, however.
Imperials can be differentiated from Bretons and Nords by their fair to swarthy skin, as opposed to the typically fairer complexion of Bretons and Nords.