Unless you're a lawyer, it's basically a distinction without a difference. Regulations are formally enacted and have the force and effect of law (much like a statute). An advisory circular is not law, but is merely one of many ways in which the FAA communicates its internal view of what the law is/requires. So, while you can't get in trouble for violating the terms of an advisory circular, per se, violating an advisory circular likely would mean that you have violated some other law or regulation, at least insofar as the FAA interprets it.
Make sense? Practically speaking, abiding by an advisory circular would mean that you have done things in the way that the FAA wants to see it, and I think it would be hard to get into trouble for doing that.
MF
