According to the books, XP includes unit testing (done by the programmers) and acceptance testing (done by the "customers"). Programmers use unit tests merely to verify that the software works as they intended. The acceptance tests are necessary to validate that the software actually works the way the "customer" wants it to.2
Specifically, the "customer" is expected to write the user stories (similar to use cases) and then write tests for the user stories. The books are a little vague on who the "customer" is, which is why I've put it in quotes. The "customer" is the person who makes the business decisions. Outside XP, this person is often called the product manager or business analyst.
