Backbone.js is a lightweight JavaScript framework for building single-page applications. It follows the MVP pattern and provides essential components like models, views, collections, and routers. Backbone.js simplifies data synchronization, event handling, and RESTful API integration. It offers a minimalistic and customizable approach, making it suitable for small to medium-sized projects. With its simplicity and flexibility, Backbone.js provides a structured way to develop applications while allowing developers to have more control over their code and architecture.