Assuming we are modeling pollution abatement (ie, a negative externality) please explain to me in words what thing(s) make up MSC and what thing(s) make up MSB.

MSC stands for Marginal Social Cost and it is the total cost to society of producing one more unit of a good or service. It includes both the private cost (the cost to the producer) and the external costs (the cost to people and the environment), such as noise and air pollution. MSB stands for Marginal Social Benefit and it is the total benefit to society of producing one more unit of a good or service. It includes both the private benefit (the benefit to the producer) and the external benefits (the benefit to people and the environment), such as less noise and cleaner air.