What is the difference between white box, black box and gray box testing?

Black box testing is a testing strategy which is based solely on the requirements and specifications. Black box testing requires no knowledge of the internal paths, structure, or implementation of the software under test. In this testing, the overall functionality of the application is checked.

White box testing is a testing strategy which is based on the internal paths, code structure, and implementation of the software under test. White box testing generally requires detailed programming skills. White box testing is mostly done by developers as they know the internals of the application. In white box testing, we do code reviews, view the architecture, remove bad code practices and component level testing.

In Gray box testing, the tester look in to the "box" under test just long enough to understand how it has been implemented. Then the box is closed and tester use his/her knowledge to choose more effective black box tests.