The Backend is the part of a web application that handles business logic, database interactions, authentication, and server configuration. It runs on the server side and is responsible for processing requests from the client, performing operations on the database, and returning responses to the client. Backend development involves working with server-side languages such as Node.js, Python, Ruby, Java, and frameworks like Express.js, Django, Rails, and Spring. The backend ensures the proper functioning of the application, including data storage, security, performance, and integration with other systems or services.