The Strategy pattern is a way to define a family of algorithms. Conceptually, all of these algorithms do the same things. They just have different implementations.
I showed an example that used a family of tax calculation algorithms. In an international e-commerce system, there might be different tax algorithms for different countries. Strategy would enables me to encapsulate these rules in one abstract class and have a family of concrete derivations.
By deriving all the different ways of performing the algorithm from an abstract class, the main module (SalesOrder in the example above) does not need to worry about which of many possibilities is actually in use. This allows for new variations but also creates the need to manage these variations—a challenge I discuss in Chapter 16.