One of the biggest complaints about Windows 8 was the missing Start menu on the desktop. Microsoft replaced it with a full-screen application menu. This new user interfaced worked really well for tablets.
Microsoft gave in, and brought back the Start menu in Windows 10. Yes, they did skip Windows 9 for several different reasons (I am not going to discuss them in this article), and lots of rumors (for example '9' sounds like 'no' in German).
Windows 10 is a mash-up of Windows 7 + 8, with several new features and other enhancements. Some of the new features include Cortana (an AI assistant), the new Microsoft Edge browser (which replaces the Internet Explorer), and a great deal more.
Microsoft is offering free upgrades to the new OS to current users (refer to the Microsoft site for the terms and conditions), and new users who are not qualified will have to buy a license. The upgrade is supposed to be available July 29, 2015.
To check if you can upgrade, check the notification area in the lower right hand corner. You will see a white Windows flag (see the graphic below)
Once you click on it, it will display the following window.
From here you can reserve your copy of Windows 10, and you can check for known incompatibilities. To check for incompatibilities, press the 'hamburger' menu in the upper right hand corner, then select option under 'getting the upgrade.' In my case it was 'your PC is ready.'
After that hopefully you will get a screen like the one below letting you know there are no hardware or software incompatibility issues.
Additional Windows 10 resources: