ICWA Question:

Marginal Cost is the amount at any given volume of output by which aggregate costs are changed if the volume of output is increased or decreased by one unity. The aggregate costs consists of both, fixed cost and variable cost. In simple words, marginal cost indicates the per unit variable cost.
Marginal Costing is on the other hand is the ascertainment, by differentiating between fixed costs, variable costs, of the marginal costs and of the effect on profit of changes in volume and type of output.