“the system of the State is composed of several apparatuses or institutions of which certain have a principally repressive role, in the strong sense, and others a principally ideological role. The former constitute the repressive apparatus of the State, that is to say the State apparatus in the classical Marxist sense of the term (government, army, police, tribunals and administration). The latter constitute the ideological apparatuses of the State, such as the Church, the political parties, the unions (with the exception of course, of the revolutionary party or trade union organizations), the schools, the mass media (newspapers, radio, television), and, from a certain point of view, the family. (…) the State repressive apparatus, the State in the classic Marxist sense of the term, possesses a very rigorous internal unity which directly governs the relation between the diverse branches of the apparatus. Whilst the State ideological apparatuses, by their principal function—ideological inculcation and transmission—possess a greater and more important autonomy”

Althusser: (from http://www.marxists.org/reference/archive/althusser/1970/ideology.htm)
In much of marxism “the State is explicitly conceived as a repressive apparatus. The State is a ‘machine’ of repression, which enables the ruling classes (in the nineteenth century the bourgeois class and the ‘class’ of big landowners) to ensure their domination over the working class, thus enabling the former to subject the latter to the process of surplus-value extortion (i.e. to capitalist exploitation). The State is thus first of all what the Marxist classics have called the State Apparatus. This term means: not only the specialized apparatus (in the narrow sense) whose existence and necessity I have recognized in relation to the requirements of legal practice, i.e. the police, the courts, the prisons; but also the army, which (the proletariat has paid for this experience with its blood) intervenes directly as a supplementary repressive force in the last instance, when the police and its specialized auxiliary corps are ‘outrun by events’; and above this ensemble, the head of State, the government and the administration. (…) the definition of the State as a class State, existing in the Repressive State Apparatus, casts a brilliant light on all the facts observable in the various orders of repression (…) while there is one (Repressive) State Apparatus, there is a plurality of Ideological State Apparatuses. (…) whereas the unified – (Repressive) State Apparatus belongs entirely to the public domain, much the larger part of the Ideological State Apparatuses (in their apparent dispersion) are part, on the contrary, of the private domain. Churches, Parties, Trade Unions, families, some schools, most newspapers, cultural ventures, etc., etc., are private. (…) What distinguishes the ISAs from the (Repressive) State Apparatus is the following basic difference: the Repressive State Apparatus functions ‘by violence’, whereas the Ideological State Apparatuses function ‘by ideology’.
I can clarify matters by correcting this distinction. I shall say rather that every State Apparatus, whether Repressive or Ideological, ‘functions’ both by violence and by ideology, but with one very important distinction which makes it imperative not to confuse the Ideological State Apparatuses with the (Repressive) State Apparatus.

This is the fact that the (Repressive) State Apparatus functions massively and predominantly by repression (including physical repression), while functioning secondarily by ideology. (There is no such thing as a purely repressive apparatus.) For example, the Army and the Police also function by ideology both to ensure their own cohesion and reproduction, and in the ‘values’ they propound externally.

In the same way, but inversely, it is essential to say that for their part the Ideological State Apparatuses function massively and predominantly by ideology, but they also function secondarily by repression, even if ultimately, but only ultimately, this is very attenuated and concealed, even symbolic. (There is no such thing as a purely ideological apparatus.) Thus Schools and Churches use suitable methods of punishment, expulsion, selection, etc., to ‘discipline’ not only their shepherds, but also their flocks. The same is true of the Family….”

Althusser summarizes this as follows:

“1. All the State Apparatuses function both by repression and by ideology, with the difference that the (Repressive) State Apparatus functions massively and predominantly by repression, whereas the Ideological State Apparatuses function massively and predominantly by ideology.
2. Whereas the (Repressive) State Apparatus constitutes an organized whole whose different parts are centralized beneath a commanding unity, that of the politics of class struggle applied by the political representatives of the ruling classes in possession of State power, the Ideological State Apparatuses are multiple, distinct, ‘relatively autonomous’ and capable of providing an objective field to contradictions which express, in forms which may be limited or extreme, the effects of the clashes between the capitalist class struggle and the proletarian class struggle, as well as their subordinate forms.
3. Whereas the unity of the (Repressive) State Apparatus is secured by its unified and centralized organization under the leadership of the representatives of the classes in power executing the politics of the class struggle of the classes in power, the unity of the different Ideological State Apparatuses is secured, usually in contradictory forms, by the ruling ideology, the ideology of the ruling class.”

Althusser asks “how is the reproduction of the relations of production secured? (…) for the most part, it is secured by the exercise of State power in the State Apparatuses, on the one hand the (Repressive) State Apparatus, on the other the Ideological State Apparatuses. (…) it is possible to represent the reproduction of the relations of production in the following way, according to a kind of ‘division of labour’.

The role of the repressive State apparatus, insofar as it is a repressive apparatus, consists essentially in securing by force (physical or otherwise) the political conditions of the reproduction of relations of production which are in the last resort relations of exploitation. Not only does the State apparatus contribute generously to its own reproduction (the capitalist State contains political dynasties, military dynasties, etc.), but also and above all, the State apparatus secures by repression (from the most brutal physical force, via mere administrative commands and interdictions, to open and tacit censorship) the political conditions for the action of the Ideological State Apparatuses.

In fact, it is the latter which largely secure the reproduction specifically of the relations of production, behind a ‘shield’ provided by the repressive State apparatus. It is here that the role of the ruling ideology is heavily concentrated, the ideology of the ruling class, which holds State power. It is the intermediation of the ruling ideology that ensures a (sometimes teeth-gritting) ‘harmony’ between the repressive State apparatus and the Ideological State Apparatuses, and between the different State Ideological Apparatuses. (…) the Ideological State Apparatus which has been installed in the dominant position in mature capitalist social formations as a result of a violent political and ideological class struggle against the old dominant Ideological State Apparatus, is the educational ideological apparatus.”

I don’t know anything about France. If I did it would help make sense of all this. I suspect Althusser is massively overgeneralizing from mid-20th century France to capitalism as such, and I suspect he is probably overgeneralizing in France too (overstating the degree of centralization of the RSA in France). Setting that aside though (ie, setting aside if and when what he says was ever the case), it seems to me that a lot of what he says is not really the case in the U.S. today. Schools are increasingly repressive, and ISAs are increasingly defunded or broken down (the decline of unions is a decline in one kind of ISA), while the RSA is expanding. I would also add that in the U.S. there’s not really one single RSA. There’s multiple RSAs and his description of the ISAs (autonomous, a mix of public and private) is true of the RSAs and has been since at least the late 19th century – for example the existence of organized white supremacist violence, the Pinkertons, etc – as well as the multiple and relatively discrete chains of command in public RSAs: there are multiple agencies at the federal, state, county, and municipal levels, as well as associations of personnel at different points in the chain of command (police unions, police chiefs’ associations, etc).

One more note and quote.
Reproduction is Althusser’s jumping off point in this essay. Early on he writes “How is the reproduction of labour power ensured?
It is ensured by giving labour power the material means with which to reproduce itself: by wages. Wages feature in the accounting of each enterprise, but as ‘wage capital’, not at all as a condition of the material reproduction of labour power.
However, that is in fact how it ‘works’, since wages represents only that part of the value produced by the expenditure of labour power which is indispensable for its reproduction: so indispensable to the reconstitution of the labour power of the wage-earner (the wherewithal to pay for housing, food and clothing, in short to enable the wage earner to present himself again at the factory gate the next day – and every further day God grants him); and we should add: indispensable for raising and educating the children in whom the proletarian reproduces himself (in n models where n = 0, 1, 2, etc….) as labour power.”