An event handler is a programming construct that allows a program to respond to specific events, such as user actions or system occurrences. When an event occurs, the event handler is triggered to execute a predefined function or set of instructions. This is commonly used in graphical user interfaces (GUIs) to manage interactions like clicks, key presses, or mouse movements. In many programming languages, event handlers are associated with objects, such as buttons or forms. For example, in web development, an event handler can be attached to a HTML element to perform actions when a user clicks on it. This makes applications more interactive and responsive to user input.