Consumer surplus: The difference between what consumers are willing to pay and what they actually pay.
Producer surplus: The difference between what the producers are willing to supply a good for and the actual price at which they sell the good.
In a graph:
In a graph, consumer surplus is represented by the area that is above the price, but below the demand curve.
On the other hand, producer surplus is represented by the area that is below the price, but above the supply curve.
Allocative Efficiency:
A market is allocatively efficient if P=MC. For a competitive market, it means that the total surplus (producer+consumer surpluses) is maximized.
Deadweight Loss:
The costs to society due to an inefficient allocation of resources. For a competitive market, there is a deadweight loss whenever the market is not at equilibrium. For example, suppose that there is a price ceiling (the government sets a maximum price that suppliers can charge), then the yellow area in the graph below is all the deadweight loss.
