Backend

Main difference between frontend and backend?
Fronted is data representation layer, backend is data access layer. And this term includes not only software, but also hardware, physical infrastructure, etc. Usually client is frontend and server is backend. Even if some presentation work is done on server for example.

In software engineering, the terms frontend and backend (sometimes written as back end or back-end) refer to the separation of concerns between the presentation layer (frontend), and the data access layer (backend) of a piece of software, or the physical infrastructure or hardware. In the client–server model, the client is usually considered the frontend and the server are usually considered the backend, even when some presentation work is actually done on the server itself.
— Wikipedia

A data access layer (DAL) in computer software is a layer of a computer program which provides simplified access to data stored in persistent storage of some kind, such as an entity-relational database.
— Data access layer

I have a learning plan to become a backend developer.