You're quite right. It is simply a digital camera connected to a telescope via a well engineered adapter. Because of the combination, i.e. taking a photo through the eyepiece of a telescope, you can get a very large effective focal length. In terms of quality have a look at -
The Teal were perched out on the trunk of a flooded tree and were at a range of about 30 yards. I think that this shot illustrates some of the colour and sharpness that you can get but also illustrates an issue (depending on your set-up) - vignetting.
It allows amateurs to get results that would require hugely expensive (and heavy) telephoto set-ups. As it is I can go bird watching and use my telescope normaly, then if I see a photo opportunity pop the camera on.