I don't have the patience or erudition to try to explain all of Althusser's connections to Lacanian structuralism or Hegelian dialectics. Neither do I have patience for much of his rather abstruse language, or for the neologisms like "overdetermination" that appear there. (That coke-head Freud invented the term "overdetermination", as far as I can tell, to describe objects in dreams that combined contradictory elements such as vampires in diapers riding motorcycles with training-wheels.)
I do have a favorable reaction, however, to things Althusser was saying in the 1962 article "Contradiction and Overdetermination". Basically this article is a call to arms against economic determinism, a bane of Marxism throughout the 20th century.
I hate economic determinism masquerading as Marxism. It makes me want to scream. I am usually confronted by the Trotskyist variant, while Althusser fought against the Stalinist version.
Economic determinism basically is a belief that social movements and beliefs are reflections of underlying economic structures. In its most sophisticated version, you get Charles Beard's "Economic Interpretation of the US Constitution" which attempted to explain the various clauses and subclauses in terms of the different economic interests of various constituencies of the American bourgeoisie. Now Beard was a Progressivist historian and didn't know any better. Stalin, a "Marxist", had no excuse when he elaborated his "3rd Period" theory, which stated that the Great Depression and the rise of fascism would create the contradictions necessary to turn the masses decisively toward revolutionary socialism. This undialectical approach had much in fact to do with the victory of Hitler and the destruction of the German Communist Party.
Althusser believed that it is a mistake to regard Marxism as a simple inversion of Hegel. If Hegel maintained that the dialectical unfolding of Ideas in history determine social relations and the state, then a simple view of Marx would tend to conclude that social relations determine ideas. Althusser is correct to point out that the relationship between social and economic relations is not "unmediated". Ideas, beliefs, customs, etc. become part of social relationships and can have as much material reality as a job or an apartment lease.
Althusser, interestingly enough, doesn't quote Marx as a counter- example to economic determinism. He cites Engels, that "distorter" of Marx who, along with Lenin, has been the favorite whipping-boy of academic Marxists for most of the century, including some folks on this list. Engels said, "The economic situation is the basis, but the various elements of the superstructure -- the political forms of the class struggle and its results: to wit constitutions established by the victorious class after a successful battle, etc., juridical forms, and then even the reflexes of all these actual struggles in the brains of the participants, political, juristic, philosophical theories, religious views and their further development into systems of dogmas -- also exercise their influence upon the course of the historical struggles, and in many cases preponderate in determing their form..."
Althusser, as I read him, is somebody who is urging the dogmatically minded--in his case, the French CP--to "go back" to Marx and Engels and forget about the one-dimensional malarkey coming from the pages of L'Humanitie. Good for Althusser.
Trotskyism and Maoism have also been a fertile ground for economic determinism. In their case, it has taken the most virulent form of "workerism". This is a belief that any social movement that does not rise directly out of the workplace at the point of production is to be suspected, if not viewed as reactionary. For a brief time, the SWP rejected "workerism" in the 1960's and 70's, but succumbed to it in the 1980's. It is the kiss of death for a socialist organization.
Althusser's project seems to be of altogether different nature than that of the anti-Marxists grouped around the Frankfurt school, but I plan to say a word or two about them in the months to come. I also want to speak about Korsch, Gramsci, Lukacs, Laclau/Mouffe, Deleuze/Guattari and others. I am determined to speak about them in plain and perhaps unflattering language. Most of what they are saying can be translated into understandable terms. Once that is done, you have the option of accepted or rejecting it. In the meantime, I give everybody permission to read Althusser.