[Matthias247]

RPC is a general term and stands for remote procedure call. You do a function call which might kind of look like a normal function call, but the actual function is executed on another host.

gRPC is one implementation of RPC, where HTTP/2 is used as a transport layer, and protocol buffers are used for data serialization. You typically use it be using the grpc framework: Generate code for a specific API, and then use the generated code and the client library to perform the call. There might however also be different ways, e.g. proxies to HTTP systems and server introspection mechanism that allow to perform calls without requiring the API specification.