2-way: The sound frequencies are reproduced by two speakers, a tweeter for the highs and a woofer for the lows.
3-way: The sound frequencies are reproduced by three speakers, a tweeter for the highs, a mid-range speaker for the mids, and a woofer for the lows.
2.5-way: The sound frequencies are reproduced by four speakers, one tweeter for the highs, one mid-range speaker and two woofers for better separation in the lows.