What is false consciousness?

The Happy Feminist gives what we might call a good working introduction to the notion of false consciousness.

In sum, 'false consciousness' as a concept may have use, even a great deal of use. Certainly 'consciousness raising' sessions among feminists in the '60s and '70s were a valuable exercise for women working through the myriad ways in which they had taken for granted their own subordination or failed to recognize ways in which the values and institutions with which they lived were operating contrary to their best interests. On the other hand, 'false consciousness' should never be treated as a one-size-fits-all response to everyone who disagrees with feminist ideas or with one's particular feminist view point.

I say a working introduction because feminist epistemologists (especially Lynn Hankinson Nelson and Helen Longino) have views on the use of `assumptions' in knowledge claims and our experience of our lives that are far more subtle and nuanced than Marx's false consciousness. If you're not a professional pedant, though, you probably don't care so much about that.