Unplanned tests that are created to conclusively detect and exploit performance issues to aid in the tuning effort. To be effective, these tests must be researched collaboratively with the technical stakeholders who have intimate knowledge of the area of the system exhibiting performance issues.
This is a "learn as you go" technique, used where a system is not well understood and not well documented, and where there is no time for extensive data gathering. The idea is to perform an iterative, evolving series of short test cycles: test the system on an exploratory basis, learn about the system through the testing, decide based on the test results where next to focus the test effort, and then move on to the next iteration of testing. No extensive, formal test plan or test cases are developed prior to starting the testing.
A technique for testing computer software that requires minimal planning and tolerates limited documentation for the target-of-test in advance of test execution, relying on the skill and knowledge of the tester and feedback from test results to guide the ongoing test effort. Exploratory testing is often conducted in short sessions in which feedback gained from one session is used to dynamically plan subsequent sessions. From RUP
See also "What is Exploratory Testing?"∞
by James Bach∞
or checkout his website∞