Evil and impurity are often referred to in the Zohar as "the other side" [sitra achra], meaning the side distinct from, and opposed to, holiness. Evil is also referred to as kelipa, meaning literally "shell" or "bark". The kelipa conceals within it a spark of holiness, which is the vital force by virtue of which the kelipa exists, analogous to a fruit surrounded by a shell or peel. In order to release the holy spark the encumbering shell must be removed.
The fourth level envelops the spark of holiness
The Zohar distinguishes among four kelipot, three of which are entirely evil. The fourth, kelipat noga, is the shell which actually envelops the spark of holiness. It thus has direct contact with holiness and is not entirely evil. The other three kelipot surround kelipat noga and are therefore further removed from holiness.
All matters pertaining to the prohibitions of the Torah (the forbidden objects, actions, utterances, thoughts, etc.) derive from the sitra achra, from the three impure kelipot. Those matters which are permitted by the Torah (but do not pertain in a direct manner to the commandments of the Torah) derive from kelipat noga.
[From the "Fiftieth Gate" edition of the Zohar, Introduction]