The "To-Do List & Diary App" is built using the React library. It allows users to create, manage, and track both their list of tasks that need to be completed and their personal diary entries. Users can easily add, edit, and delete tasks, as well as mark them as complete. In addition, they can create, update, and delete diary entries, offering a comprehensive tool for managing tasks and documenting daily thoughts and experiences. This application provides a convenient and interactive way to enhance productivity and self-reflection.