Dialectical Materialism was the official Soviet Marxist-Leninist philosophical outlook first synthesized by Joseph Stalin in 1938, though the same term had been used before to broadly refer to the more general unity of materialism and dialectics. Dialectical Materialism officiates the unique philosophical (or, one may argue, anti-philosophical) outlook of Marx & Engels, as interpreted by Lenin in Materialism and Empirio-Criticism.
According to the Dialectical Materialist outlook, nature comprises a single and inter-connected whole, whose various parts cannot be taken in isolation. It regards the content of this whole to be characterized by a unity of opposites and thus effectively a fundamental tension. This unity of opposite propels the continuous and ceaseless development of nature, never finding satisfaction in a single form.
However, the continual flux of nature, rather than constitute a single line of development, is characterized by the emergence of qualitative differences from quantitative ones. Put differently, all of the various qualitative differences in reality (i.e., different things in general) arise on the basis of quantitative changes. The continual addition or subtraction of a prior form of matter really participates in the development of an entirely new form of matter.
The Dialectical Materialist outlook regards all phenomena, all the various and different things observed in nature to be only different forms of matter, having no purely independent content of their own. In contrast to the idealist outlook, Form, rather than possess independent reality, is a particular relation of content. Qualitative differences correspond to distinct and particular relations of matter, relations which acquire distinction as forms on account of being specific determinations of the whole.
Dialectical Materialism can be contrasted to metaphysical materialism, which regards what is only a single form of matter as the supreme substance of reality. It can also be contrasted with Hegel's idealistic dialectics, which regard fundamental contradictions of nature as mere moments in the development of the Mind, elevating the faculty of thinking and ideation to the status of supreme reality.
"(Dialectical) Materialism does not focus on supreme substance (atoms/particles), it focuses on the materiality of differences and contradictions ... What gives it its difference? The difference is the Real" — Haz Al-Din