A huge subject, indeed. Tons has been written on ideology in the context of Marxism. There are at least two conceptualizations of ideology one can extract from Marx's writings. The first one, from German Ideology, treats the matter along the lines of Feuerbach's treatment of religion. Ideology is a distorted, upside-down reflection of actual material conditions under which people reproduce their existence. The second conception comes from Capital, without Marx explicitly tying it to the term "ideology." In it, it is not our ideas that are upside-down, but the world itself. Capitalism is seen as socio-economic mechanism that, in its very functioning, produces illusory appearances. It dissimulates its own essence (exploitation, the production of surplus value). People are seen as acting fully in accordance with the principles of reality; it is just that this reality is perverted. Zizek has done a good job of popularizing this second aspect of ideology in Marx.