the # of channels- the 1 is the subwoofer, which is "the bass" for the whole system since it's less directional, and then the other is the number of other channels- 2.1 is a stereo pair for higher frequencies, plus a subwoofer, etc. on up....You need to have receiver/amps/sound card and sources compatible with the speaker systemIf you have room, the Pi Ones would make *great* speakers for the 2,5, or 7 parts...or the Pi Twos, and get powered subwoofer if you don't wanna build one. If you wanna build one, ask Wayne for some plans.