Understanding Active and Passive Speakers
All speakers, regardless of the number of drivers, pole type, enclosure type, or other characteristics, fall into one of two categories: active or passive. How they're classified depends on their relationship to the amplifier driving them.
The vast majority of speakers are passive. A passive speaker doesn't have a built-in amplifier; it needs to be connected to your amplifier through normal speaker wire. This speaker level signal has been amplified enough to drive the speakers sufficiently.
Active speakers, on the other hand, have a built-in amplifier and are fed by a low-level (line-level) signal passed along an interconnect cable originating at your preamplifier or controller. Because the amplifier is an active electronic device, it needs power, and so you have to put any active speakers near a power outlet.
For most audio systems, the subwoofer is probably going to be your only active speaker (though you can also find passive subwoofers, and some high-end audio systems use these). There's no practical reason for any of your other speakers to be active.
Active speakers limit your ability to pick amplifiers tailored to your audio system and are generally expensive. They are also much harder to find. Most active speakers either fall into the low price/low-end category (designed for hooking into PCs or portable CD/MP3 players), or the really high end category (where the speakers cost several thousand dollars each).