Prototype Event observe() Method
This method registers an event handler on a DOM element.
To register a function as an event handler, the DOM element that you want to observe must already exist in the DOM.
Here are the explanations about the passed parameters:
element: The DOM element you want to observe; as always in Prototype, this can be either an actual DOM reference, or the ID string for the element.
evenetName: The standardized event name, as per the DOM level supported by your browser. This includes click, mousedown, mouseup, mouseover, mousemove and mouseout.
handler: This is the event handler function. This can be an anonymous function you create on-the-fly.
useCapture: Optionally, you can request capturing instead of bubbling. The details are in the DOM Level 2 Events
Here is an example, which observe click event and take an action whenever a click event occurs.
To understand it in better way you can Try it yourself.