What exactly is event-driven programming?

Event-driven programming focuses on the events (often called messages) and their flow between different software components. In fact, it can be found in many types of software. Historically, event-based programming is the most common paradigm for software that deals with direct human interaction. It means that it is a natural paradigm for GUIs. Anywhere the program needs to wait for some human input, that input can be modeled as events or messages. In such a framing, an event-driven program is often just a collection of event/message handlers that respond to human interaction.

Events of course don't have to be a direct result of user interaction. The architecture of any web application is also event-driven. Web browsers send requests to web servers on behalf of the user, and these requests are often processed as separate interaction events. Some of the requests will indeed be the result of direct user input (for example, submitting...