A network protocol is an agreed set of rules governing how two or more systems exchange messages. Those rules cover three things: syntax, the format and encoding of the messages (which bits go where, in what order, with what header fields); semantics, what each message and field means and what action a recipient should take; and timing, the sequencing and pacing of exchanges, such as which side speaks first, how acknowledgments work, and what happens on timeout. Because the rules are public and precise, machines built by different people in different decades can still interoperate.
The clearest primary illustrations are the foundational Internet protocol specifications. RFC 793, Jon Postel’s 1981 Transmission Control Protocol, defines TCP as a protocol that “provides for reliable inter-process communication between pairs of processes in host computers attached to distinct but interconnected computer communication networks.” The specification spells out the precise message format (the TCP segment header), the meaning of each control bit and field, and the timing behavior — the connection-establishment handshake, sequence-number-based ordering, acknowledgments, flow control, and retransmission on loss. That combination of format plus meaning plus timing is exactly what makes TCP a protocol rather than just a data structure.
Protocols are designed to be composed. RFC 791, the companion Internet Protocol specification, notes that “internet protocol interfaces on one side to the higher level host-to-host protocols and on the other side to the local network protocol,” each protocol treating the data handed to it by the layer above as opaque payload. This is what allows protocols to be stacked into a layered architecture, with each protocol responsible for one well-bounded job.
The architectural place of protocols within a layered system was formalized by Hubert Zimmermann in his 1980 OSI Reference Model paper. There, protocols are the rules governing communication between peer entities at the same layer on different machines — the horizontal agreement — while the service interface is the vertical agreement between adjacent layers on the same machine. Distinguishing the peer-to-peer protocol from the local service interface is one of the conceptual cornerstones of modern networking.
Protocols are how the open Internet is specified and governed. Rather than a central authority dictating one implementation, the community publishes protocol definitions — historically as Requests for Comments — and anyone may build a conforming implementation. The protocol document, not any single vendor’s code, is the authoritative contract, which is why precise, well-written protocol specifications have outlived the hardware and companies that first implemented them.