what's the difference? I know actives have batteries, and passives don't, but what really sets them apart?

Quote by silhouettica
Oh, DON'T use a knife. It cuts through your strings. I did that once, thinking, its the Low E, its invincible. Turns out, its not...

Quote by Kensai
actives have a built in pre amp and passives don't
Schecter C1 Hellraiser FR
1967 Ventura Les Paul Copy
Marshall JCM 2000 Dual Super Lead
I think actives also give off a warmer sustain.
Currently using:
B.C. Rich NJ Deluxe Jr. V
B.C. Rich Revenge Warlock
Bugera 6262 212 120 Watt amp :

Coming soon:
B.C. Rich Pro X Mockingbird Hardtail
Quote by TheBodomBullet
I think actives also give off a warmer sustain.

If by warmer, you mean sterile and cold, then yes. Actives give a "warmer" sustain.

Active pups are generally very consistent and are typically used for high gain applications. However, this doesn't mean that that is all they can be used for. Some apply it to jazz or even blues and rock because they have such a stable tone. The problem with the stable tone though is that you lose the subtle nuances that passive pickups give you.

Passive pups are generally lauded for being more organic than active pickups, therefor having a generally warmer tone. This is not always the case though, as some passive pickups are quite the opposite.

There are always exceptions to both cases, but the general consensus is that active pickups are very good for high gain (though they should be used for a broader range of applications too) and passive pickups have more variety, allowing one to have more choice when it comes to purchasing a pickup.
"there is no spoone" - The Matricks
Yeah..My active EMGs are slightly compressed but ballz to the walls..8 ounce shot
of steriods..Good tone..but needs left and right hand techniques..

My passive EMG's ...are weaker..but pick attack shines through...and they have a
natural violin-like vibrato...

My seymour duncans let pick attack shine through nicely as well..but they have a
natural reverb..

Active pups can create wails and exotic bends that passive cant...but the passives
dont have the preamp..so string guages and pick attack can make more of a difference because theres no compression. Compression is great for powerchords..
but the combination of octaves and string gauges can make chords sound extremely beautiful on passive pups and acoustic guitars...

Fingerstyle can be better on passives and acoustics because of the dynamics that
i discussed up above..but frantic bluegrass chicken pickin can really use compression..I always thought active pups may be better for that...

So actives are kind of like getting all of the super loud and outrageous tones..
but you have to give up the small things that really gives you that special something that people cant quite get their finger on..

Get both...You will use different ones to get your point across...lol
I bet Charlie Brown's teacher's name was Mrs.Hammett