The Brain is the body organ responsible for intelligence, personality, memory, voluntary movement and the regulation of automatic body functions such as breathing and the heartbeat. It is contained in the skull. It is connected to the rest of the body by the spinal cord. The average human brain weighs about three pounds.
As modern science can keep a body alive (breathing and heart beating) more or less indefinitely through extraordinary measures, brain death (the absence of activity on an EEG) is generally accepted to be the standard legal definition of death. However, persons with massive brain damage may still be not be classified as brain dead and may be perfectly capable of breathing on their own.
The brain is more dependent on oxygen than any other organ in the body. Deprivation of oxygen for as little as three minutes can result in irreversible brain damage. As such, early treatment of conditions that can result in loss of blood flow to the brain, such as a stroke, must be treated quickly.
The brain is isolated from the rest of the body by the blood-brain barrier, a membrane which only allows the passage of oxygen and appropriate food (usually glucose) to the brain from the bloodstream. As a result, some medicines, diseases and treatments that move through the bloodstream will not affect the brain, although some others (notably alcohol) will.
Brain damage is always the indication of a serious condition. It can be caused by trauma (a concussion), progressive diseases (Parkinson's disease, Alzheimer's disease), bacterial diseases (Syphilis) and even parasites. Often, a change in behavior or personality in a patient is a clear sign of brain damage.