A To-Do List app built with React is a front-end application that allows users to create and manage their tasks. It typically consists of an input field where users can enter a new task and a list that displays all the tasks added by the user. Users can mark tasks as complete or delete them, and the list updates accordingly. The app does not require any back-end server as it only manages tasks locally in the user's browser. It uses React state to manage the tasks and their state (e.g., completed or not completed).