Gray Box Testing

Even though you probably don't have full knowledge of the internals of the product you test, a test strategy based partly on internals is a powerful idea. This can be called as gray box testing.
The concept is simple:
If you know something about how the product works on the inside, you can test it better from the outside.

This is not to be confused with white box testing, which attempts to cover the internals of the product in detail. In gray box mode, you are testing from the outside of the product, just as you do with black box, but your testing choices are informed by your knowledge of how the underlying components operate and interact.

Gray box testing is especially important with Web and Internet applications, because the Internet is built around loosely integrated components that connect via relatively well-defined interfaces. Unless you understand the architecture of the Net, your testing will be skin deep. Hung Nguyen's Testing Applications on the Web (2000) is a good example of gray box test strategy applied to the Web.