Roc was depicted as a gigantic bird, not just large by avian standards, but literally gigantic. Some tales spoke of Roc so large it could pick up an elephant in its talons much like a regular bird of prey may pick up a young lamb.
Due to its immense size and predatory nature, Roc was a hazard to heroes of legend such as Sinbad. It is similar in some ways to the Native American legend of the Thunderbird. However unlike the Thunderbird, Roc is neither supernatural nor wrathful, though as stated above, it can pose a danger to humans due to its sheer size and predatory habits.
Roc is also classified as a cryptid alongside the Thunderbird of legend, though science would undoubtedly be looking for a creature far smaller than the elephant-snatching monster of legend.