What Is Legalism?

To pluck the mask from the face of the Pharisee, is not to lift an impious hand to the Crown of Thorns.
Charlotte Brontë, Preface to the Second Edition of Jane Eyre

All one has to do is read posts on some Christian forums to know that the labels legalism, legalist, antinomianism, and antinomian are being shot back and forth like spitballs in an out-of-control classroom. But do those using these terms really know what they mean? From what I have read on these forums, it seems many do not.