The Economist explains:  

The term skeuomorph was originally coined in 1889 to refer to an ornamental design derived from the structure of an earlier form of a particular object. In a paper on the subject Dan O'Hara, a philosopher of technology, gives the example of pottery jugs from Zaire, the handles of which are shaped in imitation of handles of traditional jugs customarily made of cord. The design of the handles thus survived the change of material. Other examples would be car seats made of plastic, but textured to imitate leather; plastic spoons moulded with patterns to provide an echo of engraved silver; or imitation wood-grain printed on furniture or flooring. Mr O'Hara argues that, strictly speaking, the term skeuomorphism refers only to those vestigial elements in nature or artefact that survive from an original form, even though they are no longer required. At its broadest, this definition extends to the levers and dials in a modern aircraft cockpit, for example, which no longer connect to systems directly, but instead are merely inputs and outputs of a computer that actually controls things. Such controls are skeumorphs because they are holdovers from the days before computerised aircraft, and have been left in their original form for the benefit of pilots, who are used to them working in a particular way.

So can a digital depiction of something properly be called a skeuomorph? It is more accurate to refer to it as a visual metaphor that calls to mind a physical skeuomorph without really being one. The iPhone's notification panel, with its imitation linen effect, was not descended from an object that was once made of linen. The switch that, say, allows you to switch an iPhone into Airplane Mode is not an on-screen replacement for what used to be a physical switch. In the early days of graphical user interfaces, designers employed familiar devices, such as folders, trash cans and other objects commonplace in the office. The result was that operating systems ended up being littered with depictions of things that had never existed inside a digital device; the on-screen "trash" icon is not the vestigial remnant of an actual trash can that was once part of the computer, which is why it is really a metaphor, not a skeumorph. (That said, as computer graphics became more detailed, the original blocky icons gave way to more detailed depictions of trash cans, folders and so forth, which are arguably skeuomorphs of visual metaphors.)