What is Extreme Programming and what's it got to do with testing?

Extreme Programming (XP) is a software development approach for small teams on risk-prone projects with unstable requirements. 
It was created by Kent Beck who described the approach in his book 'Extreme Programming Explained'. Testing ('extreme testing') is a core aspect of Extreme Programming. Programmers are expected to write unit and functional test code first - before the application is developed. Test code is under source control along with the rest of the code. Customers are expected to be an integral part of the project team and to help develope scenarios for acceptance/black box testing. Acceptance tests are preferably automated, and are modified and rerun for each of the frequent development iterations. QA and test personnel are also required to be an integral part of the project team. Detailed requirements documentation is not used, and frequent re-scheduling, re-estimating, and re-prioritizing is expected.