The problem here is that cultural relativism is a neutral term, describing a philosophical position that argues that moral and ethical values are culturally dependent. Rather than any particular value being absolutely good in itself, instead all values are constructed by a common set of cultural beliefs. Thus for us to condemn the practice of human sacrifice among Carthaginians and Incas, or cannibalism and torture among the Hurons, would simply be a misguided application of modern western values to cultures with different value systems. What many philosophers grapple with are the limits to relativism, because while one wishes to admit that certain values such as monogamy vs. polygamy are culturally relative, few of even the most extreme relativists wish to condone human sacrifice.
The problem with "essentialism" is that it is not actually a philosophical system but rather a term that many postmodernists use to insult people with whom they disagree. From a postmodernist perspective, holding any belief that there are things that are not radically contingent is to mistake accidents of time and place and cultural belief for absolute truths. To a great degree, the rhetoric of deconstruction relies on accusing anyone who opposes it of essentialism.
Cultural relativism is a perspective which holds that beliefs, ethical values, and customs cannot be held to one set standard; in other words, each belief, each custom, and each ethical value is relative to an individual's social context. Therefore, what may be considered socially wrong in one particular group of people may not be in another. For instance, in the United States, freedom of religion is a type of relativism as it embraces the right of people to practice whatever religious beliefs they hold as long as they do not break the laws of the land in doing so.
In contrast, essentialism is the view that all things and all values can be defined or described precisely. In other words, all things contain an essence, or basic quality, that is a permanent attribute which defines them. For example, the perspective of essentialism, murder by its very essence--the taking of another human being's life--is immoral, no matter in what cultural setting it is committed.