An event handler is a piece of code that listens for the browser to tell it that something is happening, and then responds to that something.

By an event occuring. Unless you are talking about the form for it, which is x.addEentListener('event', function);

A function that is used without defining it, or giving it something to be called by. i.e. I could define a function "function", and call it, or I could use the function in the middle of something, essentially defining the body of the function as it is being used.

the addEventListener() method is a means by which you can detect if something is happening to some particular DOM element, such as being clicked or loaded or what have you. The parameters: event, which is the event being listened for. function, which is what will happen upon the event happening. Finally, there is a rarely used third parameter that takes a boolean value, that determines whether the listener listens for the event outside of the specific element.