Glass is an amorphous form of silica.In glass $SiO_4$ tetrahedra are randomly joined to each other.It does not have a long range regular structure and no sharp melting point.On heating it softens and melts over a wide range of temperature.
Quartz is a crystalline form of silica.In quartz $SiO_4$ tetrahedra are regularly joined to each other and have long range regular arrangement.It has a sharp melting point.If quartz is melted and the melt is cooled very rapidly,it changes into glass