Assuming that you're talking about Windows 8 versus Windows XP or Windows 7...
I've had limited experience with Windows 8, but from what I saw the main difference was the "home screen"; meaning, the first thing you see when you turn the computer on. They implemented an app-like interface that reminded me of a tablet.
I found that new interface to be annoying and confusing, so after a few minutes I minimized it to go back to the "normal" Windows 7-type interface.
In addition, they also made the File / Edit / Help menu in Windows Explorer considerably more confusing (I don't mean Internet Explorer; Windows Explorer is the program you use to look at files on the hard drive).
Other than those two things, I didn't see any real difference between Windows 7 and Windows 8. I didn't like either "upgrade", though, so I have no intention of upgrading until it becomes necessary for security purposes.
