Materialism is the philosophical position that the material world — that is, the "natural world" of matter and energy — is the only thing that truly exists. It rejects supernatural explanations of observable phenomena, as well as philosophical idealism and spiritualism.
Materialism is the basis of all scientific inquiry, although many scientists do hold supernatural or spiritual beliefs that may even play a role in their scientific endeavors. Materialists would see other forms of energy such as light photons as the same substance as matter.
In common parlance, materialism also means the attitude of valuing material possessions over intangible things like love and honor (some strawmen would also associate this term with being a greedy immoral monster). For this reason, the word can have a pejorative connotation. Materialism should not be seen as bad. Love and honor have a physical basis in the human brain. Neural psychologists are discovering how loving brains and brains that respect honor differ from other brains. Scientists do not know the details yet. That should not stop us valuing love or honor or thinking of them as tangible and real. (Not only that, but we are after all biological life-forms who exist in the physical realm, it would be unhealthy for us, both mentally and physiologically, to reject all that exists on the sole and baseless criteria that it is "physical" - including what the strawman considers to be the only thing this term applies to)
Materialism in the philosophical sense (there is no "spiritual substance") and materialism in the common sense (material possessions are the most important thing in life) are often confused, which leads to either an equivocation fallacy or a complete misunderstanding of what skeptics believe, and in some cases has resulted in theists claiming that "atheists worship materialism". This is often tied to an equivocation fallacy regarding the word "spirituality", which can either be a reference to a real "spirit" separate from the body, or simply a set of attitudes regarding what is most important in life.
Historical Materialism is a methodological approach to the study of society, economics, and history. Historical materialism looks for the causes of developments and changes in human society in the means by which humans collectively produce the necessities of life. The non-economic features of a society (e.g. social classes, political structures, ideologies) are seen as being an outgrowth of its economic activity. This means that socio-economic and political factors cause historical change, and as a result of this some necessities in life for future generations will change comparative to past generations.
In the context of religion and theism, historical materialism shows that over time, particularly in economically developed countries since the mid 19th century, society has seen a reduction in religious participation and practice as well as an increase in atheism. Therefore it is conclusive to say that capitalist development has resulted in an increase in the plethora of ideological opinion, foremost of which being atheism and the increasing acceptance of science, as society increasingly sees that blind faith in a Deity is not a necessity as there are other paths its citizens can take to live a happy and fulfilled life.