Commonly, the majority of user machines on the web are running Windows (different versions).
Now there are several versions of this system - 95, 98, 2000, Me, NT, XP, Vista, Windows 7, Windows 8.
Macintosh is the one more general user. Very few user client machines are running any of the Unix variants or Linux, although your particular client base may consist of mainly these platforms. A lot of universities and Internet kiosks run Unix machines or terminals.
Awareness of what your user group will be using in a typical scenario will help coordinate your software testing activities.
In such a way, if your application is an online university bookstore, knowing that the universities you serve have their student computer labs filled with various Unix terminals may influence your test configurations (configuration testing).
That is why multi-platform testing is very important to the success of your project.
So if it is necessary to check the compatibility of your application with different operating systems or screen resolutions this is the most important service for your project.