Ruling gradient : It is the maximum gradient with which designer wants to design the road.
Limiting gradient : It is steeper than ruling gradient and it is provided only when there is enormous increase in cost of construction with ruling gradient. 
Exceptional gradient : It is steeper than limiting gradient and provided only, if the situation is unavoidable with stretches not more than 100m
Minimum gradient : It is provided along the length of road from drainage point of view.