the biggest difference between active and passive is where the crossing over takes place. active cross's over at the signal level, meaning before the amp with RCA's. passive cross's over at line level, meaning the speaker wire, after amps.
if you go active and have 2 woofers and 2 tweeters then you need 4 channels. passive would need only two.
Theres plenty of info on this, go read up a lil