Glasses are amorphous (non-crystalline) solid materials (most commonly silicates). The nature of the glass transition between a fluid or regular solid and a glassy phase is poorly understood.
According to Professor Philip Warren Anderson :
“The deepest and most interesting unsolved problem in solid state theory is probably the theory of the nature of glass and the glass transition.”
Through the glass lightly Science,17 March 1995.
Also see: Glass at Wikipedia.
Note: Glasses do not normally 'flow' at ambient temperatures. The much cited fact that ancient glass windows are often thicker at the bottom is explained because glaziers fitted them that way - heavy-end downwards.