A proposition is a sort of sign that signifies something. An argument is a sign structure that takes propositions into a certain form to signify a conclusion. The argument objectifies the conclusion, that is in bringing the propositions together in a particular structure it brings to a mind some further proposition other than those in the argument.
Logic then is the science of structuring proposition-signs (Pierce calls these dicisigns) so that the proposition signified by the argument (suadisign) proceeds necessarily from the original. Fallacies then are failures to signify with this sort of necessity.