There are 2 mian types of speaker, Active and Passive
An active speaker has the amplifier onboard, usually meaning it will cost more.
A passive speaker is driven by the amplifier it is plugged into.
Active subwoofers are more common now as with home cinema systems, people maybe able to plug their speaker set up directly into their TV's, which may not have the amp to drive Passives, which would usually be plugged through an amp/reciever