Brainwave entrainment or 'brainwave synchronization', is any practice that aims to cause brainwave frequencies to fall into step with a periodic stimulus having a frequency corresponding to the intended brain-state (for example, to induce sleep), usually attempted with the use of specialized software. It purportedly depends upon a 'frequency following' response on the assumption that the human brain has a tendency to change its dominant EEG frequency towards the frequency of a dominant external stimulus. Many softwares exist where you can create your own brainwave entrainment patterns. A good example would be the Neurovector Program.

Binaural Beats 
Binaural Beats deserve special mention because of the manner in which the desired frequencies are obtained. Brainwave entrainment may be achieved when audio signals are introduced to the brain causing a response directly related to the frequency of the signal introduced, called binaural beats. Two tones close in frequency generate a beat frequency at the difference of the frequencies, which is generally subsonic. For example, a 495 Hz tone and 505 Hz tone will produce a subsonic 10 Hz tone, roughly in the middle of the alpha range. The resulting subsonic tone may affect the state of mind of the subject. The 'carrier frequency' (e.g., the 500 Hz in the example above), is also said by some to affect the quality of the transformative experience.  Note that this effect is achieved without either ear hearing the pulse when headphones are used. Instead, the brain produces the pulse by combining the two tones. Each ear hears only a steady tone. 

Monaural Beats
Monaural Beats are the result of the arithmetic (vector) sum of the waveforms of the two tones as they add or subtract from one another, becoming louder and quieter and louder again.

Monaural and binaural beats are rarely encountered in nature, but in man-made objects, monaural beats occur frequently. For example, two large engines running at slightly different speeds will send 'surges' of vibrations through the deck of a ship or jet plane. The lower pitched tone, is called the carrier and the upper tone is called the offset.

Monaural beats occur in the open air and external to the ears. For example, when two guitar strings of slightly different frequencies are plucked simultaneously. Monaural beats strike the ear as beats and therefore excite the thalamus, an action crucial for entrainment. Binaural beats played through loudspeakers become monaural beats.

To hear monaural beats, both tones must be of the same amplitude. However binaural beats can be heard when the tones have different amplitudes. They can even be heard if one of the tones is below the hearing threshold. Noise reduces the perceived volume of monaural beats whereas noise actually increases the loudness of binaural beats. Good examples of monaural beat programs can be found here or here.

Isochronic Beats
Isochronic tones are regular beats of a single tone used for brainwave entrainment. Similar to monaural beats, the interference pattern that produces the beat is outside the brain so headphones are not required, but since isochronic tones are more pronounced, the stimuli is even stronger. They differ from monaural beats which are a sine wave pulse rather than entirely separate pulses of a single tone.

Isochronic tones work by the contrast between the 'on' and 'off' settings in the noise. because off is nothing, and on is on, there is a high level of contrast. this contrast causes your brain to change its thought patterns. Good examples of isochronic beat programs can be found here.