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 (the late) 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."

Source : 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.