An event handler is a programming construct that allows a program to respond to specific actions or events, such as user interactions. When an event occurs, like a mouse click or a key press, the event handler executes a predefined function or block of code to handle that event. This mechanism is essential in creating interactive applications, especially in web development. In many programming languages, event handlers are associated with objects, such as buttons or forms. For example, in JavaScript, you can attach an event handler to a DOM element to perform actions when the user interacts with it. This makes applications more dynamic and responsive to user input.