Karl Marx
Karl Marx was a philosopher, economist and one of the founders of the Internationale. In collaboration with Friedrich Engels he gradually developed the foundations of dialectic (and historical) materialism.
It is impossible to summarise this theory on a webpage, however, the following short quotation from Marx indicates the main thrust of dialectic (and historical) materialism:
The general conclusion at which I arrived and which, once reached, became the guiding principle of my studies can be summarised as follows. In the social production of their existence, men inevitably enter into definite relations, which are independent of their will, namely relations of production appropriate to a given stage in the development of their material forces of production. The totality of these relations of production constitutes the economic structure of society, the real foudnation, on which arises a legal and political superstructure and to which correspond definite forms of social consciousness. The mode of production of material life conditions the general process of social, political and intellectual life. It is not the consciousness of men that determines their esistence, but their social existence that determines their consciousness. At a certain stage of development, the material productive forces of society come into conflict with the existing relations of production or - this merely expresses the same thing in legal terms - with the property relations within the framework of which they have operated hitherto. From forms of development of the productive forces these relations turn into their fetters. Then begins an era of social revolution. The changes in the economic foundation lead sooner or later to the transformation of the whole immense superstructure. (Marx: Preface to The Critique of Political Economy)
You can read Marx's works here.