Power bands are mystical bands that are the source of the Guardians power that gives them armored suits and powers when activated. They are worn around the Guardian's arm and the Guardian must say "Empower the (animal they are the Guardian of)!" to activate it. The power bands come in six colors: red, green, white, blue (sometimes shown as purple), green and black; according to Wu Xing (The Five Elements)
The true source of The Guardian Powers are from The Powerbands' Gems; these are small, trapezium shaped objects that are placed on top of every Powerband. Without them, The Powerband would not work, and if they are damaged- they could cause the users powers to go haywire.
Other people can wear and transform in another Guardian's power band.
When a Guardian is defeated in battle, their power band's power transfers to the winner.
The black power bands should be blue because their element is water and in Wu Xing water is blue, however, the only blue power bands are with Shadow Guardians and the Guardians with black power bands are not Shadow Guardians so that may have led to confusion if they had the same colored power band.
There are probably other Shadow Bands such as the Shadow Rabbit, Shadow Ram, Shadow Rooster and Shadow Pig, however, this cannot be certain as these Bands and Guardians have never been shown on the show.
The gem in the centre of the band always corresponds to the promotional picture of The Chinese Zodiac.
The gem in the centre of the band is where all the powers are held, not the powerband itself.