Cultural materialism is an anthropological framework which relies on interpretation of material goods as indicative of greater societal values. It was introduced in 1968 by Marvin Harris, who expanded upon the earlier work on cultural ecology by Julian Steward. Where cultural ecology interprets material goods, beliefs, and behaviors as "traits" immediately determined by the environment, cultural materialism as outlined by Harris recognized the interconnected nature of multiple aspects of culture.
Harris outlined three levels of society which may be evidenced in or interpreted from material goods. First there is the superstructure, the overarching and guiding beliefs, values, and norms of a society. There is also the infrastructure, the means by which goods and the population are produced and reproduced. Negotiation between the superstructure and infrastructure is achieved through the structure- the governmental, social, and economic powers which help to organize a society. Cultural materialists primarily study the relationship between these three levels and how it contributes to the reproduction of a society.
One of the strengths of cultural materialism is that it is a framework which values quantification. If we are considering a particular culture and find that their means of reproduction- say, subsistence farming of a staple crop- is directly tied to their artistic values- perhaps through art which celebrates the staple crop- we could theoretically measure the relationship between the two. Cultural materialism is especially valuable in measuring change. In fact, cultural materialism is heavily based on Marxist studies, which deal with the radical change in the relationship between the superstructure and infrastructure.
Quite famously, Marvin Harris wrote on the sacred status of the cow in India. To many outsiders, it may seem strange to warrant an animal sacred status- especially one regularly raised for the purpose of consumption in the Western world. The famines on India prompted many outsiders to ask, "Why don't they just eat the cow?" The assertion that the cow is sacred, holy, and an Earthly manifestation of a god is a good answer, but it neglects to address how the cow attained such a high status. The truth is, cows are much more valuable in the agricultural system of India if they are kept alive (for milk, fuel, and labor) rather than killed for a few meals of beef. Because they are so useful when kept alive, their presence is quite vital in the traditional Indian agricultural system. They are sacred for their utility (superstructure,) and so are not eaten (infrastructure.)
One could argue that the cultural materialist value on quantification also presents its biggest weakness- it often neglects the emic perspective. While it's all well and good to measure crop yields and how this influences greater behavioral patterns, it says little about the experience of the individual who must adjust their behaviors. Harris valued both the etic (quantitative, observed) and emic (qualitative, experienced) perspectives, but much of the writing on cultural materialism which followed him tends to leave out insider perspectives.