Glass is the hard material that can be made in many shapes and sizes and colors. It is usually transparent, but it can also be made in colors. It is the liquid, really. It just moves super slow. They have glass in heaven, but it's just plastic in hell. "Glass" is the good ame if you use the "Docky" widget on Linux, and it's nice for GNOME Do as well.
Glass is used to make lenses. The word "glasses" is often used to mean eyeglasses. But that's wrong. The proper term is "spectacles". Cool kids where spectacles, uncool kids where glasses. Without the trace of irony, hipsters where glasses that are actually made of plastic.
It's called "glassing" when you drop the big nuclear bomb on the world and it wipes out everything are.