EEG (Electroencephalography) technology is used to measure brain's electrical vibrations from the surface of the scalp. The resulting EEG pattern will contain frequency elements mainly below 30Hz. The frequencies are categorized into four states as follows:
Delta

0.5Hz - 4Hz

Deep sleep
Theta

4Hz - 8Hz

Drowsiness (also first stage of sleep)
Alpha

8Hz - 14Hz

Relaxed but alert
Beta

14Hz - 30Hz

Highly alert and focused

The dominant frequency in the EEG pattern determines what shall be called the current state of the brain. If the amplitude of the alpha range frequencies is highest, then the brain is said to be in the alpha stage. Note, that other frequencies still exist and it is impossible to give any "exact frequency your brain is operating on". However, later references to the brain states use the simplification of assuming that such a single frequency exists.

Stimulating the brain with binaural beat frequencies
The easiest way of applying stimulus to the brain is via ears. Other senses could be used as well, and vision is actually used quite often (often in addition to hearing). However, humans cannot hear sounds low enough to be useful for brain stimulation, so special techniques must be used. One such special technique used is called binaural beats .

If the left ear is presented with a steady tone of 500Hz and the right ear a steady tone of 510Hz, these two tones combine in the brain. The difference, 10Hz, is perceived by the brain and is a very effective stimulus for brainwave entrainment. This 10Hz is formed entirely by the brain. When using stereo headphones, the left and right sounds do not mix together until in your brain. The frequency difference, when perceived by brain this way, is called a binaural beat .

Meditation is essentially about willingly being able to alter one's brain frequency to a desired state. While meditators have traditionally used several years to learn the techniques of meditation, you can now attain the same effect with brainwave entrainment. No special training or great discipline is required. Good meditation frequencies are in the alpha range, from 8Hz to 13Hz.

When a certain brainwave state is experienced and practiced over a period of time, the brain will "learn" the state change and it will become easier to self-produce the desired brainwave state at will. Thus, using brainwave entrainment, you can expect to get some of its effects later even without any entrainment.