Electroencephalography (EEG) is the measure of the electrical activity that your brain generates. When brain cells, or neurons, communicate with one another, voltages are produced by the movement of ions in and out of neurons. This is similar to the electrical activity of your heart. However, neurons in the brain are frequently activated in large groups and at different frequencies. The synchronous activity of these large groups of neurons generate ‘brainwaves’ that are recorded on the scalp during an EEG. Brainwaves can change for a number of reasons including when a person gets drowsy, falls asleep, is resting, in deep thought, or for pathologic reasons including after a concussion.
For more information please see this scholarpedia article.