Backend and frontend
Front and backend are the options of software architecture.
The terms appeared in software engineering due to the development of the principle of separation of responsibility between the external presentation and the internal implementation of the program.
Frontend is what the user sees; the side of the user interface to the hardware and software part of the service. In other words, it is a beautiful, convenient and user—friendly "shell" of the program.
Backend — the hardware and software part of the service; what is "under the hood".
The backend implements APIs used by the frontend to provide its function. Therefore, the frontend developer does not need to know the specifics of the server implementation, and the backend developer does not need to know the implementation of the frontend.