LEGO has created two sets of collectibles with the Bionicle line (not counting the trading cards by Upper Deck): Kanohi and Krana. These masks and "face brains" come in different shapes and colors, and can each be attached to the standard head that comes with most sets. Kanohi are "good" (except for the Infected Hau), and Krana are "bad".
The source of a Toa's power is their Kanohi (which translates as "face"). This mask of power can be either a Great Mask of Power or a Noble Mask. Each mask has a different name and power. Toa generally use Great Masks, and Turaga/Matoran use Noble Masks, although some Tohunga use Great Masks. Each mask has a different power, and each color is worn by a different individual.
Mask packs, sold for approx. US$2-3, each contain 2 masks and a head, eyes, and connector rod for display. 72 standard masks are packed randomly, including an infected Hau. A "mystery mask" pictured on the side of the box has turned out to be the Vahi Mask of Time, which is distributed mainly as a promotional item.
Twelve "European misprint" masks were accidentally produced and distributed for a short time in Europe, at the very beginning of the Bionicle production run. Due to miscommunication between departments, masks in the wrong color were run and sold. A full set of these 12 masks typically sells on eBay for more than US$200, on the rare occasions they are available.
There are currently 117 masks released. The rarest, and not really considered part of the "collection," is a solid 14kt gold Hau produced for promotional giveaways (contests, drawings, etc.) It weighs 1.1 oz. And it is rumored that only 30 were produced. Each gold mask is worth approximately $800.
Gold and silver-colored Great Masks were introduced into Krana packs at the same time as the Bohrok Swarm.
The Bohrok Swarms invaded Mata Nui just after the Toa defeated Makuta. Carrying the "power brain" Kranas behind their head shield, and resupplied by smaller Bohrok Va, the Bohrok began terrifying the island. Their sole purpose: destruction of the island on behalf of Cahdok and Gahdok, twin queens who have power over the Bohrok and Krana.
Krana may be flung by a Bohrok, and if it attaches to another face (Toa, Turaga, Matoran), can take over that being.
The Krana brains are rubbery rather than plastic, and fit over the "skull" headpiece common to most Bionicle sets. Krana and Kanohi cannot be worn at the same time.
There are eight different types of Krana, and each comes in 12 different colors, for 96 different Krana to collect.
See Krana checklist for full list.