Passive pickups are a magnet assembly and coil(s) of wire. They can be wired up "passively" so they just go through a volume control and tone control before the output jack. Passive pickups can also be wired through an onboard preamp.
Acive pickups, and the most common are the EMGs, are a similar magnetic structure, ie magnets and coils, but built into the case of the pickup itself is a small preamp so you get nothing out of the pickup unless it's connected to a battery. Active pickups can also be wired simply through a passive volume and tone pot, or through am onbard preamp.
Like all things, both active and passive pickups, and onboard preamps all have their positive and negative attributes and the variations on sound out of all of them is way to large to say "actives are generally...." or "passives are generally...."