A herd is a bunch of animals — or people who act like a bunch of animals. It's also a verb — when people herd animals, they try to keep them moving in the same direction.
A herd usually refers to a group of animals with hooves, like cows or sheep (meat-eating animals like wolves cruise around in packs). On a farm, sheepdogs are in charge of herding the herd of sheep. Herds of buffalo roam the plains. It's rarely a good thing when people act like a herd, which is to do what everyone else is doing. You might want "to break away from the herd" if everyone is wearing pinwheel hats, for example. If you're having trouble getting people together, you might say it's like "herding cats." Cats never follow directions.