http://www.emginc.com/guitar.asp
there are differences of opinion on this one. basically the way actives work is they use far lower magnets and there are less coils arround the poles in the pickups. this gives a much weaker signal than passive pickups. this signal is then boosted by the battery to about that of a passive pickup.
because of the low magnetic force actives have slightly better sustain. also due to the low number of coils there is less wire for the current to travle through so there is less resistance. this means there is less treble cut of (as this is affected by resistance more than the base). this gives a 'better' quality sound.
but due to actives few coils and low magnets they dont pick up the strings frequencies quite as well. many guitars sound good due to the wood used or neck construction etc as they slightly affect the way the strings vibrate. these affects will not be picked up by acive pickups. in fact actives sound the same on all guitars, so long as placing and type of active pickup are the same. many people love the sound the guitar imparts on the sound it gives 'soul' to it.
to sum up actives are a bit mechanical and cold. they give a good even response. passives have got more 'soul' and interestin sounds.
again it all depends what style u play. actives are probably better for mega distortion as at that level of distortion the 'soul' effect of the passives is lost anyway. they also give a good clean sound but it is a cold sound, acuraste and mathamatical. this clean is often used in the beginin of a metal song.
if u are gonna play blues actives are a no no. if u do buy active buy emg.