A backend, sometimes referred to as the server side or backend system, is the part of a software or web application that works behind the scenes and is responsible for processing data, providing resources and fulfilling requests that come from the user interface (frontend). The backend includes servers, databases and application logic that work together to ensure the smooth operation of the application.

In a typical client-server architecture, the user interacts with the front end (client), which usually consists of a user interface. When the user performs an action or makes a request, this request is sent to the backend, which processes, retrieves or stores the corresponding data. The backend then sends a response to the frontend, which displays the information received or presents it to the user in some other way.

Developers focussing on the backend often work with server technologies, programming languages (such as Python, Java, Ruby or Node.js) and databases (such as MySQL, PostgreSQL or MongoDB) and need to ensure that the application is scalable, secure and performant.