That tiny useless handle on your maple syrup jug, the fake spokes on your hubcaps, that shutter clap sound on your digital camera, and all the carefully rendered buttons and sliders on your computer and apps that reference the look of real-world stuff, those are skeuomorphs. In other words, "a derivative object which retains ornamental design cues to a structure that was necessary in the original." (via wikipedia).
Skeuomorphism is rampant in the digital world. The paperclip icon in email that signifies "attachment," even the whole idea of a desktop on which there are white paper documents and folders (depicted with those little extended tabs where you would write what's inside them) and pictures of the wife and kids. It's all visual metaphor, and it's all representative of some deep human need for continuity (and nostalgia) in technology.
Steven Poole has spoken out against this digital skeuomorphism. Personally, as retro as my aesthetics are, I'm in favor of new non-skeuomoprphic visual representations and haptic interfaces in the digital realms that are 100% intuitive to use.