The great auk is not related to the penguins of the southern hemisphere. But the word Penguin first was used for them. Later it was transferred to southern penguins because of their similar look and way of life. The word penguin originally comes from the welsh words "pen" and "gwyn" and means "white head", because they have a notable white mark around their eyes (Another explanation would come from some latin word i can't remember right now). Otherwise their back is black and their front (downside if swimming) is white.