truing

it is the process of changing the shape of the grinding wheel as and when it becomes warn from original shape
it is done to make the wheel true and concentric with the bore or to change the face contour of the wheel for form grinding.

dressing: it is the process of removing the loading of the wheel and breaking away of the glazed surface so that sharp abrasive particles are again presented to the work
A tool known as star-dresser us generally used to perform this operation