Grey matter looks reddish gray on a freshly removed brain. It forms the superficial parts of the brain and the deep parts of the spinal cord. It is composed of the bodies of the nerve cells (neuron) and the initial parts of its processes (axons and dendrites) just emerging from the neurons. Gray matter is the major part of the nervous system in which the nerve impulses for all kinds of mental functions are produced and then sent away to be carried to their target organs by white matter.
Generally, gray matter can be understood as the parts of the brain responsible for information processing; whereas, white matter is responsible for information transmission.
Not to be confused with dark matter (matter that doesn't emit light).