Extreme programming, or "XP" takes certain simple practices that are known to improve software quality. Then, it takes these to logical extremes.
1. Requirements are addressed by having a customer on-site, available to the programmers. If this is not possible, the customer may describe use cases on cards, that are periodically photocopied for the engineering staff.
2. Simple code is more likely to work. Therefore, extreme programmers only write code to meet actual needs in the present project.
3. Code reviews are good. Therefore XP has programmers program in pairs, sharing one screen and keyboard.
4. Testing code is good. Therefore, in XP, test suites are written before the code is written. The code is considered complete when it passes the tests. The system is periodically, or immediately tested using all preexisting automated tests to assure that it works.