Your page is delivered from the server to a client browser, somewhere out there in the Internet. The browser has drawn the page on a screen, and somebody — or some thing — is looking at it. It's a waiting game. Eyes shift back and forth, taking in this or that detail in quick jumps, darting to the side now and then, away from the screen, to investigate distractions in the environment. The clock ticks. The page glows softly, passively, as the user hovers inactive, hand loosely draped over a mouse, neck bent down and eyes more and more intent on something inviting that your page has to offer.

Suddenly, without any warning at all, the cursor begins to move as the hand on the mouse stiffens slightly and begins nudging the little plastic bump over the rough surface of the table. As the mouse moves, its surrogate on the screen moves in close imitation, grazing past interesting images and witty remarks in the content of your page. Eventually a decision is made, the movement pauses, a muscle or two contract slightly, and the mouse button is depressed by an insistent finger. The microswitch in the mouse triggers an electronic impulse, and suddenly the browser is made aware of what's happened: a mouse click.

In all that, everything about what the user has done while gazing at the page has happened in a way totally unpredictable to the browser, to any client code in your web page, to anything resident on your servers. There was no knowable "wait time" between human actions. The actions, therefore, as transmitted by the equipment hooked to the user's computer, happened when they happened and not according to a predictable clock — that is, they happened asynchronously.