Harpies were monstrous bird-like women. Traditionally, there were three or four of them and they were held responsible for anything that could not be found or had somehow disappeared. In Homer, they carried people off to their death, but later they mainly snatched away food.
They seem originally to have been wind spirits. In any event, their name means 'snatchers', from harpazo, 'to carry off, 'snatch up' (compare Harpalyce). The English words 'rapt' and 'rapture' are related to the Greek word that gives their name.