Mylar, or biaxially oriented polyethylene terephthalate is a very shiny film. Applications include packaging, laminating, insulating, reflecting all sorts of light, creating useful acoustics, and a whole load of other random uses. It looks a bit like tin foil, though it has a different consistency. In the 60s NASA used mylar in the making of certain satellites, though they used a metalized form. Mylar is also known for it's ridiculous smoothness; in fact most applications require the film to be artificially roughened so as to make handling possible.
Though I won't bore you with specific details on production, the process of making mylar is quite ingenious. Basically, the film is a bunch of molecules crystallized into locked positions, kind of like polarized sunglasses. To make it shiny, a thin sheet of metal is evaporated onto the film. If successfully created, mylar can reflect up to 99% of all light that hits it, making it ideal for solar panel and pothead applications.
All in all, quite a useful substance.