What is the Difference Between Backend and Frontend?

Think of a product as an iceberg: the visible part is the Frontend, and the unseen part that carries the system is the Backend.

Frontend

The screens and interactions the user sees: design, perceived speed, flows, usability.
Technologies: HTML, CSS, JavaScript; React, Vue, Angular

Backend

Business logic, database, security, user authentication, APIs and integrations.
Technologies: Node.js, Python, .NET, Java; MySQL, PostgreSQL, MongoDB

The Bridge: API

The user performs an action (Frontend), the request goes to the Backend via the API, the work is done and the result returns to the Frontend. In modern systems, the right architecture makes this communication fast, secure and observable.