I agree that this is easier to understand than semiotics, assuming that I'm understanding it correctly. I found Althusser's section to be most helpful. He breaks down ideology into two parts; there is the Repressive State Apparatus and the Ideological State Apparatus. He refers to the RSA as belonging to the "public domain" and functioning through violence. Constrastingly, he defines ISAs as being part of the "private domain" and consisting of religious, educational, familial, etc... institutions. The ISA, he claims, functions by ideology. He then goes on to explain the inter-relationship between the two. RSA's function mainly through violence and repression and secondarily through ideology. The ISA functions inversely.

Althusser also offers a thesis, which for me could work as a definition. His thesis is that, "Ideology represents the imaginary relationship of individuals to their real conditions of existence." His second thesis is that ideology also has a material existence but I think that this can be absorbed into the first thesis, if I'm understanding it correctly. I think that ideology is the imaginary relationship of individuals to their real conditions but it does have a material existence in that is always exists in practices.