When the cutting edge of a grinding wheel takes a glass-like appearance due to wear of abrasive grains then it is called glazing of the grinding wheel. Glazing decreases the rate of grinding. The glazing in the grinding wheels takes place when the wheel is too hard or it revolves at a very high speed.
Thus the glazing may be decreased by using a softer wheel or decreasing the wheel speed.
The process of changing the shape of grinding wheel as it becomes worn due to breaking away of the abrasive and bond is called truing.
The process of improving the cutting action of the grinding wheel is called dressing.

DressingThe purpose of dressing is to restore the correct cutting action of the wheel. Dressing removes the clogs on the surface of the wheel and the blunt grains of the abrasive, exposing the new sharp abrasive grains of the wheel which can be cut and brought to shape efficiently.
TruingTruing refers to the shaping of the wheel to make it run concentric with the axis. When a new grinding wheel is mounted, it must be trued before use. The cutting surface of a new wheel may run out slightly due to the clearance between the bore and the machine spindle. Grinding wheels, which are in use, also can run out of true, due to uneven loading while grinding.
Dressing and truing are done at the same time using same tool.