Grey box testing is a software testing technique that uses a
combination of black box testing and white box testing. Gray
box testing is not a complete black box testing, because the
tester does know some of the internal workings of the
software under test. In grey box testing, the tester applies
a limited number of test cases to the internal workings of
the software under test. In the remaining part of the grey
box testing, one takes a black box approach in applying
inputs to the software under test and observing the outputs.

grey box tesiting is a combination of both black box and 
white box testing. The tester who has programming knowledge 
can perform grey box testing.

Grey box Testing is the new term which evolved due to the 
different architectural usage of the system. This is just a 
combination of both Black box & white testing. Tester 
should have the knowledge of both the internals and 
externals of the function.

Grey box testing is the combination of black box and white 
box testing. Intention of this testing is to find out 
defects related to bad design or bad implementation of the 
system. 
In gray box testing, test engineer is equipped with the 
knowledge of system and designs test cases or test data 
based on system knowledge. 
For example, consider a hypothetical case wherein you have 
to test a web application. Functionality of this web 
application is very simple, you just need to enter your 
personal details like email and field of interest on the 
web form and submit this form. Server will get this 
details, and based on the field of interest pick some 
articles and mail it to the given email. Email validation 
is happening at the client side using Java Scripts. 

In this case, in the absence of implementation detail, you 
might test web form with valid/invalid mail IDs and 
different field of interests to make sure that 
functionality is intact. 

But, if you know the implementation detail, you know that 
system is making following assumptions 
•	Server will never get invalid mail ID 
•	Server will never send mail to invalid ID 
•	Server will never receive failure notification for 
this mail.

So as part of gray box testing, in the above example you 
will have a test case on clients where Java Scripts are 
disabled. It could happen due to any reason and if it 
happens, validation can not happen at the client site. In 
this case, assumptions made by the system are violated and 
•	Server will get invalid mail ID 
•	Server will send mail to invalid mail ID 
•	Server will receive failure notification
Hope you understood the concept of gray box testing and how 
it can be used to create different test cases or data 
points based on the implementation details of the system.