Backend is the part of a website or application that handles data management, business logic, and communication between the frontend, other applications, and databases. It is responsible for providing an interface between the user and the server-side database. The backend includes server-side languages such as JavaScript, PHP, Ruby, Python, and Java, as well as databases like MySQL and MongoDB. Backend development involves writing code to create APIs (Application Programming Interfaces) that allow other applications to interact with the application.