Glass can be made and used in many different ways to adjust its properties. Normal glass is fairly brittle and can be broken easily by striking or dropping it. However, treated glass can even be bullet proof. The transparency or translucence of glass can be adjusted. It can be so clear that birds don't see it and fly into windows or people accidentally attempt to walk through closed sliding glass doors. But it can be colored or shaded so that light passes through, but you can't see through it -- useful in glass construction blocks. Or it can be completely opaque, not even allowing light through.
Normally glass is smooth, but when hot enough (depending on the type) it is liquid and very easily moldable. It's commonly made of sand, soda ash, and lime. On the other hand, tempered glass can be used in hot, temperatures for cooking and baking. (It is usually a bad idea to heat a glass object unevenly; that may break it.)