A Backbone developer specializes in the Backbone.js library, used for building single-page web applications. They work on creating, testing, and maintaining web applications using the Backbone.js framework. Their responsibilities include developing user interface components, connecting the application with web services, ensuring high performance and responsiveness, and implementing a robust set of services and APIs to power the front-end. They also work on the optimization of applications for maximum speed and scalability, and ensure high-quality graphic standards and brand consistency. They often collaborate with other team members and stakeholders to deliver a seamless user experience.