Windows 8 was heavily criticised for apparently being designed to rival Android (and Apple's iOS) for use on mobile devices with touch-screens (such as tablets and phones) rather than being easy to use with a mouse and keyboard. Instead of a Start button, users found their screen full of apps. The most popular download for Windows 8 very quickly became an app which made Windows 8 look more like Windows 7 and reintroduced the Start button.
Windows 8.1 seeks to address some of the complaints by, for example, reintroducing the Start button for users who want to have it available. It also trys to iron out some ofthe (many) problems that were experiencedwith Windows 8.
This might be of interest: