A Full Stack developer is someone who can work on both the Front-end and Back-end of a web application. The Front-end, also known as the client-side, is the part of the application that the user interacts with, such as the layout and design. The Back-end, also known as the server-side, is the part of the application that handles the logic and database interactions. A front-end developer specifically focuses on the client side, while a back-end developer specifically focuses on the server-side.