Marginal costing is a costing technique where the marginal cost, or variable cost, is charged to units of output, while the fixed cost for the period is written off against the contribution. The marginal cost is the additional cost of producing one more unit, which can be calculated as the total variable cost per unit. Marginal costing focuses on contribution per unit and considers variable costs as product costs, while absorption costing treats both fixed and variable costs as product costs. Marginal costing is used to determine profit based on contribution and the break-even point, while absorption costing presents costs in a conventional manner for financial reporting.