Interpellation, according to Althusser, is a hailing by social ideologies that form 'individuals' into 'subjects' by forcing social identities upon them. This primarily occurs through Ideological State Apparatuses (ISA), including the family, education and mass media. It is through these ISA's that people are given both a sense of identity and an understanding of reality.
Ideology, for Althusser 'represents the imaginary relationship of individuals to their real conditions of existence' (cited in Stevenson 1995: 37). Ideology transforms human beings into subjects, leading them to see themselves as self-determining agents when they are in fact shaped by ideological processes.
This ideology comes almost exclusively from Ideological State Apparatuses. These are institutions that, according to Althusser, interpellate individuals into either the bourgeoisie (ruling class) or the proletariat. "Ideology 'acts' or 'functions' in such a way that it 'recruits' subjects among the individuals (it recruits them all), or 'transforms' the individuals into subjects (it transforms them all) by that very precise operation which I have called interpellation or hailing, and which can be imagined along the lines of the most commonplace everyday police (or other) hailing: 'Hey, you there!'" (Althusser, Ideology and ideological state apparatuses, 1971) The fact that we do not recognise this as being "ideological" shows the power of ideology.
It is so ingrained in us that we cannot see it unless we are knowledgeable about it.
Althusser emphasizes that the process of interpellation allows the existing power structure of capitalist society to maintain its domination over the general population without resorting to violence or force. Interpellation occurs in subtle ways through the workings of what Althusser calls "Ideological State Apparatuses," including official culture and such specific institutions as churches and schools, though it is constantly backed up by the physical force represented by the "Repressive State Apparatus" of the police...