A guitar cab is a passive speaker or set of speakers that capture the powered signal from an amplifier, usually connected via a speaker cable.
Do guitar cabs need power?
In short, no. As the amp head is the preamp and power amp for the cabinet. So without the amp head attached, they're just speakers.
Why do some guitar cabs have an open back?
Cabs with an open back mean the speakers have access to unlimited amounts of air, so the speaker cone can move a lot more freely. Resulting in refined lower frequencies.