The economic practice (the historically specific mode of production, currently capitalism) transforms raw materials to finished products deploying human labour and other means of production in interactive webs. The political practice does the same using social relations as raw materials.
Finally, ideology is the transformation of the way that a subject relates to his real-life conditions of existence. The very being and reproduction of the social base (not merely its expression) is dependent upon a social superstructure. The superstructure is "relatively autonomous" and ideology has a central part in it.
America's social superstructure, for instance, is highly ideological. The elite regards itself as the global guardian and defender of liberal-democratic and capitalistic values (labeled "good") against alternative moral and thought systems (labeled "evil"). This self-assigned mission is suffused with belligerent religiosity in confluence with malignant forms of individualism (mutated to narcissism) and progress (turned materialism).
Althusser's conception of ideology is especially applicable to America's demonisation of Saddam Hussein (admittedly, not a tough job) and its subsequent attempt to justify violence as the only efficacious form of exorcism.
People relate to the conditions of existence through the practice of ideology. It smoothes over contradictions and offers false (though seemingly true) solutions to real problems. Thus, ideology has a realistic attribute - and a dimension of representations (myths, concepts, ideas, images). There is harsh, conflicting reality - and the way that we represent it both to ourselves and to others.
"This applies to both dominant and subordinate groups and classes; ideologies do not just convince oppressed groups and classes that all is well (more or less) with the world, they also reassure dominant groups and classes that what others might call exploitation and oppression is in fact something quite different: the operations and processes of universal necessity"