The brain conist of billions of cells called neurons that can produce little bursts of electricity, passing from one cell to another. These bursts produce weak electric fields at the surface of the head that can be recorded.
For example electric fields that are larger and slowly-varying indicate sleep, whereas smaller and faster-varying fields indicate alertness. Electrical activity increases in certain regions of the brain when people remember, speak, or blow their nose.
The brain controls the "lower" or involuntary activities such as heartbeat, respiration, and digestion - these are known as autonomic functions. The brain also controls "higher" order, conscious activities, such as thought, reasoning, and abstraction. The human brain is generally regarded as more capable of these higher order activities than any other species.