I'm not going to try and describe what nanotubes are, aside from saying that they are very thin (1/50,000th the width of a human hair) man made [usually] carbon based tubes (fullerene "buckyballs" is another phrase that describes them, if that helps.) If you'd like to know a whole lot more about them, the Wikipedia article might be a good start.
Nanotubes are on this blog because researchers have apparently discovered that nanotubes cause mesothelioma in mice. I hasten to add that fiberglass causes mesothelioma in mice when injected into the pleural cavity, as was done in this test, and fiberglass certainly doesn't seem to cause mesothelioma in humans.
Nanotubes have potential uses ranging from aerospace applications to clothing, again according to Wikipedia , and a web site devoted to web sites devoted to nanotubes. The problem is, of course, that mesothelioma doesn't develop in humans for 20 years (or more) after exposure to asbestos. Will development of this technology be delayed while we wait and see, or will people just take sensible precautions?