Black-box testing focuses on the behavior and external instances of the software. It is directed to the expected behavior of an application or program from the user’s point of view.
White-box or glass-box testing is aimed at software with knowledge of internal data structures, physical logic, algorithms and architecture at the program code level from the developer's point of view.
White-box testing, as well as black-box testing, is an integral part of the whole software testing process. Individually, they do not provide extremely good results for balanced work of software testing company. Black-box testing can be less effective in retrieving data-flow or boundary condition errors at the source level. As for the white-box testing, it is less effective in finding macro-level errors in the operating system and errors, associated with the compatibility, usability.
Gray-box testing is a combination of white-box and black-box testing. Gray-box testing is designed to evaluate the project in terms of the interaction of its individual components.
The gray-box testing goes mainly with the testing of web applications because it considers high-level development, operating environment, and compatibility conditions. During black-box or white-box analysis it is harder to identify problems, related to end-to-end data flow. Context-specific problems, associated with web site testing are usually found during gray-box verifying. Web applications contain a lot of elements, both software and hardware. These components must be verified in the context of system design to estimate their interaction and functionality.
Gray box testing consists of tools, aimed at understanding the properties of the application and the environment with which it interacts. This approach can be used in black-box testing in order to increase the productivity of testing, finding bugs and bugs analyzing performance.
On the other hand, the gray box testing is the use of incomplete or suspected information about the structure or design for further concentration or enlargement of black box testing.
Remember, that software testing is more effective with good understanding of the system behavior and its architecture, software development, requirements of the applications, types of bugs you are looking for. You must understand how to control the interactions between system components in the testing and production environments.