A full-stack developer is responsible for creating, managing, and optimizing all components of a software application. They handle backend development using languages like Python or Java to manage databases and servers, and frontend development using languages such as HTML, CSS, and JavaScript to build the user interface. They ensure seamless data flow between backend and frontend, solve design challenges, and provide end-to-end solutions. They also test and debug the entire system, and sometimes they may manage project specifications and requirements. Their ultimate goal is to create a functional, user-friendly, and efficient application.