?
According to http://www.imperial-library.info/content/seven-fights-aldudagga, Nords were once the same as giants, but time passed, and they became separate races, presumable sometime during the Dawn or the early Merethic. Things like imps and ogres have been theorized to have come from the same stock as man and mer, so giants probably came from it as well.
In ESO, there is a http://www.uesp.net/wiki/Lore:Lyris_Titanborn who claims to be half-giant, which would confirm giants to be another race of mortal, but her claims are debatable as her father is a known Nord, which would have to make her mother the giant, but since offspring takes the race of the mother, she is clearly lying, or the giantish of which she speaks comes from farther up a generation. Though she would no longer be able to claim being half-giant then. ?I've never played ESO, so they might have expanded upon it more ingame than what is given on the wiki.
Regardless, there are probably at least differing races of giant the same way there are differing races of man and mer.