In rudimentary form, it [the dialectic] states that Social Condition A (thesis) leads to an opposite Social Condition B (antithesis), thereby creating a tension leading to its resolution in Social Condition C (synthesis). Thus, Marx theorized that capitalist exploitation of workers (A) would lead to revolution and a classless society (B) followed by dictatorship of the proletariat (C).

Alan Stebbins, Robert. "Discovery." The SAGE Encyclopedia of Qualitative Research Methods. Thousand Oaks, California. Sage Publications. 2008.

... a system of dialectics argues that an existing element (or thesis) contains within itself inherent contradictions that unwittingly create its opposite (its antithesis). The result is a conflict between the two that ultimately results in the emergence of a new element (the synthesis), but this new element also contains its own internal contradictions, causing the process to begin anew....

... For Marx, dialectical contradictions have their origins in the relationships between social and economic classes. The beginning point of analysis, therefore, must be the economic circumstances within which humans find themselves.

Herod, Andrew. "Marxism, Geography and." Encyclopedia of Human Geography. Thousand Oaks, California. Sage Publications. 2006.

Dialectic

Hegel thought that all logic and world history itself followed a dialectical path, in which internal contradictions were transcended, but gave rise to new contradictions that themselves required resolution. Marx and Engels gave Hegel's idea of dialectic a material basis; hence dialectical materialism.

Singer, Peter, "Dialectic." Honderich, Ted (ed.). The Oxford Guide to Philosophy. New York. Oxford. 2005. Page 212.

Dialectical Materialism

[Negation of the negation] is sometimes presented by expositors of "dialectic" as "thesis-synthesis-antithesis"; this jargon, however, is not a characteristic of dialectical materialists. Since it was never used by Hegel, and was used by Marx only once, solely for the purpose of ridicule, it is easy to understand why its use is nearly always a sign of either ignorance of or hostility to dialectical thinking–usually both at once.

Wood, Allen, "Dialectical Materialism." Honderich, Ted (ed.). The Oxford Guide to Philosophy. New York. Oxford. 2005. Page 213.