There are two main changes. Firstly, the conventional and much missed Start menu of Windows 7 has returned, enhanced so it can display tiles and launch apps. Secondly, the different variants of Windows (for PC, tablet and smartphone) are being consolidated into something more consistent which will, nevertheless, perform equally well on any platform.
So why no Windows 9? Microsoft's explanation is that it is so different from Windows 8 that it is really two versions on. We have an alternative theory: the Mac operating system is OS X (where X stands for 10) and by calling it Windows 10 they are implying parity with what many people consider to be the trailblazer. Just a thought...