Ideology is a key term in literary, cultural, and film studies. Todays understanding of the term Ideology is rooted in the writings of German philosophers Karl Marx and Friedrich Engels. In their original sociological analysis, they defined Ideology as "The ideas of the ruling class are in every epoch the ruling ideas The class which has the means of material production at its disposal, has control at the same time over the means of mental production." The entirety or the system of ideas of the ruling class would be the Ideology of a given society. The function of ideology would be the continual reproduction of the means of production and thereby to ensure the continuous dominance of the ruling class. Ideology achieves this by distorting reality. While in fact the split in ruling and subservient social classes is artificial (i.e. man made) and serves the needs of the economic system, the ideas of ideology makes it appear natural. It makes the subordinate classes accept a state of alienation against they would otherwise revolt. This state of alienation has also been referred to as "false consciousness".
The concept of ideology as formulated by Marx and Engels has been critiqued and expanded to adapt to the changes our societies underwent since 19th century. The most obvious and fundamental critique would be that we have moved in most Western societies from an industrial economy to a service economy and thus the assumption of a social division into a ruling class that controls the means of production and a working class that is forced to sell its labor in order to survive no longer applies. Cultural critics have suggested alternative categories other than social class in which hierarchical power structures are also at work. The more obvious of those are gender, race, ethnicity, and nationality. In those contexts, ideology would work analogous to Marx and Engels model to maintain the existing power relations of, for example, a patriarchal society.
Another crucial extension of the concept stems from Italian Marxist philosopher Antonio Gramsci. He has introduced the concept of hegemony by which he means that ideologys power derives primarily from consent as opposed to the use of force. Secondly, Gramsci argued to expand ideology from a set or formal ideas to include "common sense." By the latter he means habitual attitudes which have been assimilated from ruling class ideas, i.e. they appear completely natural or commonsensical, yet they originally came into being as artificial concepts that served the purpose of a specific social group. The French critic Louis Althusser built on this concept to articulate two types of "apparatuses" for maintaining dominance: "repressive state apparatuses" (RSAs) (the army, police, etc.) which have explicit agendas to exert control and "Ideological State Apparatuses" (ISAs) (the media, education, the family) which often function semi-independently and without explicit intent to exert control. Ideology, along with discourse, are the two central concepts in understanding the cultural transmission of ideas, values, and assumptions. A key point of contention in this developing concept is how much ordinary people can resist, negotiate, or refashion ideology on their own, or whether only cultural critics armed with analytical tools for unmasking the normalizing functions of ideology and discourse can successfully play this role.
The significance of ideology for film studies primarily relates to the question whether a film (consciously or unconsciously) promotes or subverts dominant ideology, and how the discourse of classical Hollywood cinema, including its formalist features, transmits ideology. Stereotypes used in films may further the support of dominant ideologies, while practices within counter-cinemas may challenge them, especially those practices that create the effect of distanciation, which disrupts the normalizing formalist and narrative functions of mainstream cinema.
 Marx and Engels, The German Ideology quoted in Peter Brooker, A Concise Clossary of Cultural Theory
Adopted in part from Brooker, A Concise Clossary of Cultural Theory