Business processes involve interactions among autonomous partners. We propose that these interactions be specified modularly as protocols. Protocols can be published, enabling implementors to independently develop components that respect published protocols and yet serve diverse interests. A variety of business protocols would be needed to capture subtle business needs. We propose that the same kinds of conceptual abstractions be developed for protocols as for information models. Specifically, we consider (1) refinement: a subprotocol may satisfy the requirements of a superprotocol, but support additional properties; and (2) aggregation: a protocol may combine existing protocols. In support of the above, this paper develops a semantics of protocols and an operational characterization of them. This supports judgments about the potential subclass-superclass relations between protocols, which are a result of protocol refinement. It also enables protocol aggregation by splicing a protocol into another protocol.