Several years ago, philosopher Harry Frankfurt released his brief essay On Bullshit through Princeton University Press. The basic idea was that bullshit was different from a lie. A liar knows (and cares about) what the truth is and attempts to hide or distort it. Bullshitters, on the other hand, are more interested in persuading without any regard for the truth. The rhetoric could be true or false, but the only thing that truly matters to a bullshitter is that the audience is convinced. In short, liars conceal the truth. Bullshitters (sometimes) conceal their disinterest in the truth.
You can see Frankfurt discussing this concept in the fairly new1 video below.