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

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

Passive Sub:

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

Active or Powered Sub:

This type of sub is a self-contained speaker and amplifier. The speaker and amplifier are matched to for optimal performance and both are housed in the same enclosure. This 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.