A callback function is a key concept in programming, especially in languages like JavaScript. It refers to a function that is passed as an argument to another function and is executed after a specific event or task is completed. Callbacks are widely used in asynchronous programming to handle operations that don't return immediately, such as API requests, reading files, or setting timeouts. For example, in JavaScript, you might pass a callback to a function that makes an HTTP request. The function will initiate the request, and once the data is returned from the server, the callback function is executed to process that data. This allows the program to continue running other tasks without waiting for the request to complete, making the program more efficient. Callbacks can be either synchronous or asynchronous. Synchronous callbacks are executed immediately once the task is done, while asynchronous callbacks, like those found in JavaScript's event-driven nature, are executed later when a certain event occurs. Callbacks are also foundational in functional programming, allowing you to pass behavior (functions) as data. While callbacks provide great flexibility, they can lead to issues like 'callback hell,' where nested callbacks become difficult to manage and read. To solve this problem, modern JavaScript introduced Promises and async/await syntax to make asynchronous code cleaner and easier to follow. Understanding callbacks is essential for any programmer, as they play a crucial role in building responsive, efficient applications that handle multiple tasks at once.