From 1d4chan

Giant-kin is a term from Dungeons & Dragons which refers to a number of races that are related to the Giant races, but which are much smaller in size. The precise makeup of the races that consist of giant-kin is malleable, with most players referring to the following list:

In Advanced Dungeons & Dragons, the splatbook "Giantcraft" gives a more official explanation for the giant/giant-kin divide. This book is technically based in the Forgotten Realms setting, but due to how interconnected AD&D's settings tended to be, it's generally accepted that this info holds true in Greyhawk as well. According to this book, the term "giant" only refers to those races who originate from the union of the giantish father-god Annam and the giantish mother-goddess Othea. This covers hill giants, fire giants, frost giants, storm giants, cloud giants, stone giants, and the "offshoot" races of ettin, titan, fog giant and mountain giant. The term "kin" refers to the breeds that originate from Othea cuckolding Annam with the god Ulutiu, and consists of the firbolgs, fomorians, verbeegs and voadkyn. Ogres are not "technically" kin, but as the bastard children of Othea and Vaprak, they are considered little better; cyclopses are of unknown ancestry, so "true" giants reject them as another of Othea's bastard races, and no mention is made of the position of trolls in this family tree - trolls weren't formally considered a kind of giant until 3rd edition.