Each sefira, introduced in our discussion of the Tree of Life (http://www.byzant.com/kabbalah/treeoflife.html), has its unbalanced aspect; and these are said to form the infernal tree of the qelippot ("harlots" or "shells" - singular, qelippa). This tree may be viewed as growing down from Malkuth of Assiah (see the discussion of the Four Worlds (http://www.byzant.com/kabbalah/worlds.html)) in an inversion of the Divine Tree. Thus the unbalanced qelippa corresponding to Malkuth stands above that which corresponds to Kether - matter as master of spirit, an inversion of the natural order of Creation.
The qelippotic equivalent of the supernal triad, Kether, Chokmah and Binah, consists of Tohu (the Formless), Bohu (the Void) and Chashek (the Darkness) respectively. The remaining seven qelippot correspond to seven hells. On the plane of Atziluth, the qelippot have no influence, and so there is no qelippotic equivalent of the Name of God. There is, however, qelippotic influence in the three lower worlds. In Briah, the Archangel of each sefira has a corresponding Devil of the qelippa; in Yetsirah each Angelic Host is matched by a Cohort of Demons; and in Assiah each Mundane Chakra has a corresponding Infernal Habitation.
It is important to make the distinction between true and apparent evil. Geburah may be considered "evil" in its cruel, destructive aspects; but the true essence of Geburah is vital as a balance to the otherwise wanton inclusiveness of Chesed. It is an error to view one half of a dynamic duality as "good" and the other "evil": both are necessary for balance, to prevent the true evil of imbalance. It is precisely the unbalanced aspects of each sefira that are to be found in the qelippot. The qelippotic equivalent of Geburah really is pure destructive cruelty, not as a balance to anything else, but solely for its own sake. Thus the qelippot represent true, active evil, and it is part of the Great Work to neutralize such forces, not by suppression or subjugation, but by bringing them into balance as aspects of their corresponding sefirot.
One way of considering the formation of the qelippot is to see them as a direct consequence of the "Lightning Flash of Creation" in which the sefirot were emanated in order, one from the other, from Kether down to its fulfillment in Malkuth. At the formation of one sefira, and before the subsequent emanation of its successor, that sefira was in a sense unbalanced. For example, with Chokmah formed but not having yet emanated its balancing successor Binah, this unbalanced state of Chokmah created a pattern around which the qelippa Bohu could later crystallize.
Finally, it is worth noting briefly that the qelippot can also be viewed as being accessed through the "sefira which is not a sefira", Da'ath (http://www.byzant.com/kabbalah/sefira.asp?number=--1). This "sefira" has no number and no position on the Tree in relation to the other sefirot, though when it is shown it is located centrally in the Abyss (between the planes of Binah-Chokmah and Geburah-Chesed) and with no explicit connection to any other sefira. Da'ath is Knowledge, but consider the use of Knowledge without Understanding (Binah).