Sennheiser HD 280 Pro headphones@ 2004/09/15
That is, their little sound-making drivers are suspended in space close to the ears, surrounded by an ear-cup that generally plays some part in the sound of the headphone, but is basically just an open framework to hold the driver in the right place and protect it from damage.
You could take the drivers out of a lot of open headphones and suspend them next to your ears on a piece of coat-hanger wire, and you'd still get much the same sound.
Some open headphones are really obviously open - Sony's sci-fi MDRF1s are a standout. But you can see light through the ear-cups of many open 'phones if you turn them around a bit.
Acoustically, all open ear-cups are very much the same; the only big differences between them (he said, smiling benignly at the headphone designers who're now angrily waving thick wads of technical documentation at him) are where exactly they hold the drivers relative to your ears. The idea of the open design is to make ear-cups that have pretty much no acoustic signature of their own, so you can listen to the driver by itself, not the driver plus the resonance of the chunk of plastic and metal over your ear.
As you'd expect, though, open headphones let sound in, and out. Outside noise will make it to your ears, and your music will leak out into the outside world. There's no way around this, and it makes open 'phones unsuitable for use in a lot of situations. Various workplaces, public transport, monkey-houses, and, of course, noisy computer rooms. You may have a lovely pair of Beyer DT 880s, or Sennheiser HD 650s, or Stax exotics, and that's great - but only when there's not much ambient noise, or anybody nearby who doesn't want to share the treble portion of your program material..................
Comfort-wise, these are as good as any circumaural sealed 'phones I've used. Which is to say, they don't float on your ears like fluffy clouds, but I nonetheless don't mind wearing them for hours on end (unlike certain ear-squishing Beyers I could mention).