API

HTTP (Hypertext Transfer Protocol)

an application layer protocol forming the foundation of data communication for the World Wide Web
a request–response protocol in the client–server model
Request methods: Read-only : GET, HEAD, OPTIONS, and TRACE

REST or Representational State Transfer

A single URI to access via various actions/HTTP verbs (GET/PUT/POST/PATCH/DELETE)
It requires to write the requests for each type of entity in your database, in contrast to GraphQL.
Not as space efficient as RPC.

RPC or the Remote Procedure Call

more space efficient than REST
no special syntax like REST
only used for internal communication (say remote login)

GraphQL

single request fetch all needed backend data and modify at front end
more front-end development work, no backend dat aaggregations