In a review of Bad Science Val Jones finds a very useful distinction, which I am sure will serve me well the rest of my life.
The liar knows and cares about the truth, but deliberately sets out to mislead; the truth-speaker knows the truth and is trying to give it to us; the bullsh*tter, meanwhile, does not care about the truth, and is simply trying to impress us.
Ben goes on to quote the philosophy professor Harry Frankfurt (from his 1986 essay “On Bullsh*t”):
[The bullsh*tter] is neither on the side of the true nor on the side of the false. His eye is not on the facts at all, as the eyes of the honest man and of the liar are, except insofar as they may be pertinent to his interest in getting away with what he says. He does not care whether the things he says describe reality correctly. He just picks them out, or makes them up, to suit his purpose.