Brainwave Frequencies - These are frequencies associated with various mental states. Using brainwave entrainment, you can coax your brainwaves to a certain frequency, and in doing so, achieve the mental state associated with that frequency.

Brainwave Entrainment - This is using stimulii (such as sound, light, or EM fields) to influence the rate of our brainwaves, which in the process affects our mental state.

Our brain plays "monkey see/monkey do" with stimulii in the proximity. If you have lights flashing at 4 cycles a second, our brain will begin to mimic that with its own brainwaves. When our brainwaves are at a certain frequency, our body will do whatever it normally would do -- we normally produce brainwaves of 4 cycles a second when we're sleeping - so pulsating light at this rate would put a person to sleep.

Say you want to do this with sound instead of pulsating light - since most brainwave entertainment frequencies are below 30 HZ, and are often below human hearing range, you (usually) can't just generate the tones using a wave generator. There are two methods that are typically used to generate these ultra-low tones in a way that humans can perceive them and feel their effects :
1) Modulation - While a tone at 4 HZ would be too low to hear by itself, if you generate noise and then fluctuate its pitch or volume at 4 times each second, this can also be used for brainwave entertainment, and would affect us the same way a 4 HZ binaural would. CoolEdit from Syntrillium Softwareis the best software to use with this method. (Although, he uses a slightly more complicated technique than just fluctuating the pitch or volume.)

As far as which of these two methods are better - binaural beats probably work better for brainwave entertainment, but the second method will work from speakers - headphones aren't required. That can come in handy - especially when you want to experiment with brainwave entertainment while sleeping or meditating - ever tried falling asleep wearing headphones? I have a cordless pair, and even with those, it's not that comfortable.

(And if that's not enough information about Brainwave Entertainment, check out the Brainwave Entertainment links for even more.)

2) Binaural Beats - Using a computer or synthesizer (and a good pair of stereo headphones), generate two tones, one in the left ear, one in the right - your brain imagines it hears a third tone equal to the difference between the two tones. (For instance, if you had a 400 HZ tone in one ear, and a 404 HZ tone in the other, you would perceive a third tone of 4 HZ.) And this would have the same effects that a pulsating light at 4 cycles per second would. In order for this to work, the two tones need to be below 1000 HZ. There are two good programs for creating Binaural Beats : Cool Edit from Syntrillium Softwareand Brainwave Generator. The latter is probably the most user-friendly, and is good for people new to brainwave entertainment, since it has presets that come with it. (I found it to work very well, since it incorporates a strobe light into the mix as well) .