What is the difference between an active subwoofer and a passive subwoofer?

What's a subwoofer?

A subwoofer (sub) is a speaker dedicated to the reproduction of low-pitched audio frequencies most commonly referred to as bass.
 

What's a passive subwoofer? 

Passive subs are called passive because they need to be powered by an external amplifier or Audio/Video (A/V) receiver. It's similar to traditional speakers. The critical thing to remember about a passive sub is that it needs more power to reproduce low-frequency sounds. Therefore, the amplifier or A/V receiver needs to be able to send enough energy to sustain bass effects reproduced by the sub without draining the power supply in the A/V receiver or amplifier.

What's an active or powered subwoofer?

This type of sub is a self-contained speaker and amplifier. It stands on its own. The speaker and amplifier are matched for optimal performance, and both are housed in the same device. This method takes a lot of the power load away from the A/V receiver and allows the receiver or amplifiers to just power the mid-range and tweeter speakers.