The iPad introduced  a new UI element to iPhone OS – the popover.  Here’s how Apple describes it:

You can use this iPad-only view to temporarily display additional information, controls, or choices related to content in the main view. The main benefit of the popover is that it can contain information or choices that don’t need to be in the main interface all the time.

Much like the familiar overlay on the Web or dialog on the desktop, popovers can be modal or non-modal. One thing that distinguishes the iPad popover from others is that it has an arrow that points to the area where the user invoked it.

Popovers are very flexible and can be used as palettes, menus, hierarchical browsers and more. In fact, it’s interesting to note just how similar the iPad popover is to an iPhone app. They are roughly the same size and shape and they follow the same conventions. In the example below, you can see that the iPhone fits neatly over a Keynote popover.

Here is a typical popover shown in the Keynote app for iPad:

Keynote with popover

And here is the same keynote screen with an iPhone superimposed over the image:

iPhone over iPad popover

Interesting, isn’t it? Below are a few more examples of popovers that should give you a good sense of how flexible this element really is.

Bento Library Popover

Bento's Library popover uses a coverflow interface.

Editable popover in Things

An editable popover in Things

A hierarchical popover in Twitteriffic

A hierarchical popover in Twitteriffic

You may be thinking, “well, duh,” but this is a stroke of genius in my mind. Think about it. Apple could have gone any number of ways with the UI. They could have copied the desktop or they could have invented an entirely new UI construct. Instead, they logically extended the iPhone interface whose interaction model, size, shape and (importantly) scalability have been proven out over the last few years. This is the true test of great design. It scales and extends gracefully from the simple to the more complex.