He is the economist most responsible for the field of transaction cost economics. Transaction costs are the costs of an economic exchange. As an example, if you decide to buy a car, the price of the car is one of many costs you have to incur. You also have to research to find what kind of car you want, spend time driving to dealerships or looking through classifieds to find the car you want, and finally spend time negotiating on the price or terms of the deal. All of these are transaction costs. These type of costs are many times ignored in the theoretical realm of economic models, but Williamson brought attention to their effect.
Here is a review of one of Williamson's better known works by a UCLA law professor. He adds this assertion on Williamson and transaction costs:
Williamson's core idea is the theory of transaction cost economics. We can analogize transaction costs to friction: they are dead weight losses that reduce efficiency. They make transactions more costly and less likely to occur. Among the most important sources of transaction costs is the limited cognitive power of human decisionmakers.
Can you think of any other examples where transaction costs are important other than when buying cars? Are there cases where transaction costs can cause you to not make a purchase that you would make if they did not exist?