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. For this reason, metaphysical explanations are usually rejected out of hand.
- "When you go to a car mechanic, do you go to one that does a séance to rid your car of evil spirits? Or do you go to a car mechanic that looks for real and physical reasons that your car doesn't work? "
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 is sometimes accused of reductionism, which over simplifies reality in an attempt to explain it. In particular, it rejects metaphysical explanations which theists claim are necessary to explain the divine. 
"But in the real world our inwards are invisible."
- — Lady Reason in The Pilgrim’s Regress by C. S. Lewis
However, materialism is arguably not reductionist if it is consistent will all real phenomena. There is no reliable evidence of any phenomena that is incompatible with materialism.
"The reasons for which "this" [material] world has been characterized as "apparent" are the very reasons which indicate its reality; any other kind of reality is absolutely indemonstrable."
Critics of materialism claim that it supports moral relativism, i.e. "all points of view are equally valid." , since it rejects objective morality.  However, this is a false dichotomy since these are not the only two positions on the matter. Also, this is a straw man argument because materialism also rejects this form of moral relativism since it is also a metaphysical position. While materialism sees personal values as a product of culture, upbringing, environment DNA, etc., it does not claim that different values are "equally valid" because "valid" in this context is a metaphysical concept.
Undermines personal responsibility
Materialism also supposedly undermines personal responsibility because it claims that external factors can influence our decisions and our "free will". While factually questionable, it is also an appeal to consequences - the desirability or undesirability of the consequences of a belief are not factors in determining its validity.
Utopianism is an attempt to create a perfect world and is another alleged consequence of materialism. This movement is dangerous because it does not consider or allow for practical limitations, compromises with other groups and the often uncooperative aspects of human nature. In other words, utopianism is another form of reductionism.
"The utopian dream of a perfect society and a perfect human being, the idea that we are moving towards collective salvation, is one of the most dangerous legacies of the Christian faith and the Enlightenment. Those who believe in the possibility of this perfection often call for the silencing or eradication of human beings who are impediments to human progress. They turn their particular good into a universal good. They are blind to their own corruption and capacity for evil. They soon commit evil, not for evil's sake but to make a better world."
While there is an element of truth in this argument, particularly for totalitarian governments, this argument is largely a straw man attack on the separate modernist belief that humans can improve their condition without relying on God. Just because a person works towards making the world a better place, it does not automatically imply a belief in a perfect world. Improving the world by human effort alone can be problematic for some theists because it flies in the face of religious fatalism.
Unweaving the rainbow
The poet Keats complained that "cold philosophy" was "unweaving the rainbow" by "conquer all mysteries". While scientists have explained many previously mysterious phenomena such as earthquakes, lightning, optics, etc., it has uncovered a vast array of deeper mysteries that are even harder for humans to understand. Also, we should be wary of valuing ignorance for the sake of aesthetics. This argument is taken up in the book by Richard Dawkins of the same name.
Confusion with seeking material possessions
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.