Dialectical materialism is most sharply contrasted with idealism, both explicit and implied, in the sense of the objective existence of ideas fundamentally independent of and distinct from the material reality of (intuitively) rocks and trees or (scientifically) the intrinsic, irreducible properties of quanta and quantum fields.
Examples of explicit idealism can be found in Plato's philosophy as well as Hegel's dialectical idealism, the direct precursor to Marx's dialectical materialism. A more subtle, implied form of idealism can be found in many people's — including many scientists — ideas about natural physical laws, the idea that physical laws have an independent existence from material reality. There exists a law of gravity, and the law of gravity itself actually exists independently of actual material things attracting each other. (To be fair, most practicing scientists do not worry deeply about or get too attached to the fine details of philosophical ontology, nor do they really need to do so. For their practical purposes ontological fuzziness is just as useful as informed skepticism.)
When we start talking about specifically political philosophy, the distinction between idealism and materialism becomes very important. According to the precepts of dialectical materialism, we have to re-think 99% of our discourse on political and ethical philosophy as at the very least using unwarranted and confusing idealistic metaphors and at worst being arrant bullshit.
Much of Western political and ethical philosophy consists of the search for the "correct" ethical and political principles. What are the correct set of property rights? How much freedom of speech should we have? Who should own the means of production? When do the needs of the many outweigh the needs of the few?
From a materialist viewpoint, these questions are at best metaphorical and at worst nonsensical. Ethical and political principles are not in any sense material entities (except as they exist as arrangements of neural properties in people's heads) and have no properties, such as correctness or goodness, independent of some physical, material representation. These questions can be interpreted as different kinds of metaphors: What kinds of property rights will lead to some desired outcome? What are the historical material causes of the collection of ideas in people's heads and in law books that we refer to in the abstract as "freedom of speech"?
Metaphor is fine for literature. An explicitly, consistently and more-or-less precisely defined metaphor becomes an unobjectionable label. But political philosophy is not literature, and its use of ill-defined, imprecise and poorly-understood metaphors does little to advance our understanding.
Paul Krugman gives us an excellent recent example of using an idealistic metaphor in political philosophy:
I’m also fairly conventional on how economies should be run. Self-interest is still the best motivator we know – or more accurately, the only consistent motivator. So I’m for market economies. But I’m for market economies with strong safety nets, with adult supervision in capital markets, with public provision of goods the private sector does badly (like basic research and much of education.) An idealized New Deal is about as far as I go.If you construct "self-interest" in a reasonable, materialistic way, Krugman is correct that it is indeed the "best" motivator. I'm pleased that he's "for" strong safety nets, adult supervision and public provision of some goods. But how does self-interest motivate these activities? If he thought self-interest (or material factors) could motivate safety nets, etc., he wouldn't introduce these ideas with a "but". They are idealistic principles, presumably, that we "should" aspire to independently of and in some sense contrary to* our self-interest.
*Independence is vacuous if it cannot be contrary. If two things are always correlated, they are not actually independent, even if we can somehow speak of them separately; if two things are always correlated, then speaking of one is always speaking of the other. I cannot, for example, talk (informally) about satiating hunger "independently" of eating food... at least not until we invent neural implants that can directly affect our subjective mental states. And even then, I cannot talk even formally about my feelings of hunger independently of the neural states that represent those feelings.
We can see another example of Krugman's idealism in his book, The Conscience of a Liberal. Just the title betrays his idealism: the principles he expounds are matters of conscience, to be adopted because they are themselves "good" principles. He extols the virtues of New Deal regulated imperialist finance capitalism. As capitalism goes, New Deal capitalism was pretty good, and pretty good even for the capitalist class. He talks in a very abstract, indirect way about its instrumental goodness, but mostly in terms of other ideals, such as stability and distribution of income. He can't really talk in detail about the material effects of New Deal capitalism, since New Deal capitalism offered only the most marginal benefit in terms of the measurable, material well-being of billions of people: tens of millions of Americans and Europeans and a billion people in the exploited third word living in desperate poverty. (You're now just malnourished instead of actually starving to death! Yay capitalism!)
Furthermore, he does not talk about the causal history of the New Deal, and he doesn't explore deeply the causal history of the thirty-year erosion of the New Deal from the 1970's to the present. I'm not pretending I can read his mind, but I get the impression reading the book that he believes the New Deal sprang Athena-like from the mind of FDR and (to some extent) John Maynard Keynes, and its erosion was due to nothing more than inexplicable, irrational perversity.
Dialectical and historical materialism (if we take historical materialism to be dialectical materialism applied to political philosophy) purports to explain these phenomena in a rigorous, detailed, systematic and, above all, scientific way.