What is Cultural Materialism?

Anthropology is the study of humans. Cultural materialism is an anthropological school of thought (or "research strategy") that says that the best way to understand human culture is to examine material conditions - climate, food supply, geography, etc.
Marvin Harris explained it this way in his preface to his CULTURAL MATERIALISM - The Struggle for a Science of Culture:
"Cultural Materialism is the strategy I have found to be most effective in my attempt to understand the causes of differences and similarities among societies and cultures. It is based on the simple premise that human social life is a response to the practical problems of earthly existence... In its commitment to the rules of scientific method, cultural materialism opposes strategies that deny the legitimacy of the feasibility of scientific accounts of human behavior... Cultural materialism, with its emphasis upon the encounter between womb and belly and earth and water, also opposes numerous strategies that set forth from words, ideas, high moral values, and aesthetic and religious beliefs to understand the everyday events of ordinary human life. Aligned in this regard with the teachings of Karl Marx,
cultural materialism nonetheless stands apart from the Marx-Engels-Lenin strategy of
dialectic materialism. Condemned by dialectical materialists as "vulgar materialists" or "mechanical materialists," cultural materialists seek to improve Marx's original strategy by dropping the Hegelian notion that all systems evolve through a dialectic of contradictory negations and by adding reproductive pressure and ecological variables to the conjunction of material and conditions studied by Marxist-Leninists."
You can read the entire preface here

Rival Theories

Today there are two main rivals to the materialist approach: idealism (sometimes called "cultural" in opposition to "materialist") which says that human ideas have a stronger effect on culture than material conditions; and the biological approach - often known by the current most popular school of biological thought, evolutionary psychology, which says that human culture can best be explained through human biological evolution.
R. Brian Ferguson wrote an excellent paper that illustrates the differences between cultural materialism and its rivals:
"Materialist, cultural and biological theories on why Yanomami make war."
From the paper:
Materialist explanations see war as following self interest, an effort to maintain or improve material conditions, as argued in varieties of ecological theory and conflict-oriented approaches to sociocultural evolution...
Cultural approaches hold that war is the acting out of values and beliefs characteristic of a particular group, with explications ranging from simple listings of elicited goals to dense hermeneutic deconstructions. Biological perspectives assert that war is chosen because in our speciesí evolutionary history a penchant for collective violence enhanced the likelihood of passing along genes, directly, beyond any considerations of material well-being and regardless of cultural values. Persistence of such divergent answers for so long might lead to despair about prospects for theoretical advance, for providing some firm answers to the big question. But it is not that theoretical alternatives cannot be compared and evaluated as explanations, just that they usually are not. Much more commonly, a scholar presents one theory and dismisses or ignores the alternatives. The different theoretical currents then continue in their own, self-enclosed way.

Infrastructural Determinism

While cultural materialism recognizes the importance of ideas and Darwinian principles in human behavior, it insists that material living conditions are more important than either in explaining cultural evolution. This is called the principle of infrastructural determinism.

Marvin Harris and others

The leading proponent of Cultural Materialism was the late Marvin Harris (1927-2001), who came up with the name, and wrote classroom textbooks, scholarly books, and popular works about cultural materialism throughout his career. His best known general-reader books are Cows, Pigs, Wars & Witches and Cannibals and Kings. He was a strong advocate for a scientific view of culture, as reflected in the title of Cultural Materialism: The Struggle for a Science of Culture, published in 1979.