A subrace is a distinctive division within a race, while still being part of that same race. Subraces are not the same as ethnicity, however - Ethnic groups are defined more along the line of cultural similarities, while subraces refer strictly to divisions in a particular race. Ethic groups do fall under certain subraces.
The only race with distinctive enough differences to have actual subraces are the Faeri, though due to their varied nature, Half-Breeds can also be considered to have a sort of "subrace" structure as well.