Protocol Buffers are a Google product, and they have just entered their third revision. Protocol Buffers take the approach of providing a DSL that the generator (written in C) reads and can generate client and server stubs for over ten languages, the primary ten are maintained by Google and encompass: Go, Java, C, JavaScript for NodeJS.
Protocol Buffers is a pluggable architecture, so it is possible to write your own plugins to generate all kinds of endpoints not just RPC; however, RPC is the main use case as they are coupled to the gRPC framework.
gRPC was designed by Google to be a fast and language agnostic RPC framework, which originated from an internal project where latency and speed were of the utmost importance in Google's architecture. By default, gRPC uses protocol buffers as the method for serializing and de-serializing structured data. An example of this DSL is shown in the following example.
Protocol buffer service definition:
service Users { rpc CreateUser (User) returns (Error) {} }
message User { required string name = 1; required int32 id = 2; optional string email = 3; }