The marginal cost of an additional unit of output is the cost of the additional inputs needed to produce that output. More formally, the marginal cost is the derivative of total production costs with respect to the level of output.

Marginal cost and average cost can differ greatly. For example, suppose it costs $1000 to produce 100 units and $1020 to produce 101 units. The average cost per unit is $10, but the marginal cost of the 101st unit is $20
The Econ Model applications Perfect Competition and Monopoly emphasize the roles of average cost and marginal cost curves. The short movie Derive a Supply Curve (40 seconds) shows an excerpt from the Perfect Competition presentation that derives a supply curve from profit maximizing behaviour and a marginal cost curve.