For creatures with a central nervous system, this represents the major organ of that system. For humans and other terrestrial vertebrates, the brain is housed in the head and encased in bone.
From Wikipedia on Brain:
The brain is an organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. Only a few invertebrates such as sponges, jellyfish, adult sea squirts and starfish do not have a brain; diffuse or localised nerve nets are present instead. The brain is located in the head, usually close to the primary sensory organs for such senses as vision, hearing, balance, taste, and smell. The brain is the most complex organ in a vertebrate's body.