A group of sheep is called a drove, flock or herd. A very large group of sheep is referred to as a band or mob.
To be termed "sheep," the animal must be a year old and have produced offspring. A lamb is under a year old and has not produced offspring. A yearling is an animal that's between one and two years old and has not produced offspring. A female sheep is called an ewe, and a male sheep is referred to as a ram. There are over a thousand sheep breeds worldwide, and they're classified according to their purpose, fibers, coloring or physical characteristics.