gRPC makes remote procedure calls (RPCs) possible between a client and a server over a network.

It uses Protobuf to define the data structures (messages) and the services (functions or methods) that the client can call on the server.

So, gRPC is the system that actually allows the client to call server functions, while Protobuf is used to describe the data (messages) that’s passed between them.

In short:

Protobuf defines how the data should look (structures).

gRPC lets you call remote functions and exchange that data.