This document discusses different types of costs including fixed costs, variable costs, and semi-variable costs. It defines fixed costs as those that do not vary with output over a short period of time, variable costs as those that change directly with output volume, and semi-variable costs as being partly fixed and partly variable. Marginal costing is then introduced as a technique that analyzes costs and profits based on how they change with increases or decreases in output volume, differentiating between fixed and variable costs. The key assumptions and uses of marginal costing are also summarized.