What is the main difference between windows 8 & 8.1 while comparing with new windows10?

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Dear friends.... Now I want to know the main difference between windows 8 with windows 10... an also with windows7... What are the merits more with the windows new version? A lot of my friends are asking me the above same question in order to upgrade their operating systems in their desktops/laptops... At least I want 4 lines about the above confusion now...