XP completely revolutionizes the concept of testing by giving it equal (or even greater) priority than the code. In fact, you write the tests before you write the code that will be tested, and the tests stay with the code forever. The tests must be executed successfully every time you do an integration of the project (which is often, sometimes more than once a day).
Writing tests first has two extremely important effects.
First, it forces a clear definition of the interface of a class. I've often suggested that people imagine the perfect class to solve a particular problem, as a tool when trying to design the system. The XP testing strategy goes further than that it specifies exactly what the class must look like, to the consumer of that class, and exactly how the class must behave. In no uncertain terms. You can write all the prose, or create all the diagrams you want, describing how a class should behave and what it looks like, but nothing is as real as a set of tests. The former is a wish list, but the tests are a contract that is enforced by the compiler and the running program. It's hard to imagine a more concrete description of a class than the tests.
While creating the tests, you are forced to completely think out the class and will often discover needed functionality that might be missed during the thought experiments of UML diagrams, CRC cards, use cases, etc.
The second important effect of writing the tests first comes from running the tests every time you do a build of your software. This activity gives you the other half of the testing that's performed by the compiler. If you look at the evolution of programming languages from this perspective, you'll see that the real improvements in the technology have actually revolved around testing.
Assembly language checked only for syntax, but C imposed some semantic restrictions, and these prevented you from making certain types of mistakes. OOP languages impose even more semantic restrictions, which if you think about it are actually forms of testing. Is this data type being used properly? and Is this function being called properly? are the kinds of tests that are being performed by the compiler or run-time system. We have seen the results of having these tests built into the language: people have been able to write more complex systems, and get them to work, with much less time and effort. I've puzzled over why this is, but now I realize it's the tests: you do something wrong, and the safety net of the built-in tests tells you there is a problem and points you to where it is.
But the built-in testing afforded by the design of the language can only go so far. At some point, you must step in and add the rest of the tests that produce a full suite (in cooperation with the compiler and run-time system) that verifies all of your program. And, just like having a compiler watching over your shoulder, wouldn't you want these tests helping you right from the beginning? That's why you write them first, and run them automatically with every build of your system. Your tests become an extension of the safety net provided by the language.
One of the things that I have discovered about the use of more and more powerful programming languages is that I am emboldened to try more brazen experiments, because I know that the language will keep me from wasting my time chasing bugs. The XP test scheme does the same thing for your entire project. Because you know your tests will always catch any problems that you introduce (and you regularly add any new tests as you think of them), you can make big changes when you need to without worrying that you will throw the whole project into complete disarray. This is incredibly powerful.
This is from the second edition of the e-book Thinking in Java by Bruce Eckel
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For more information see: Kent Beck extreme testing framework