The tiled interface of Windows 8 houses apps. Technically, an app is a type of software. But you'll often hear the two terms used as if they mean different things. In Windows 8, there's a big difference.
In general, an app is a piece of software that is relatively easy to install and use. Most apps have a fairly limited set of features. Part of that is to keep the app's file size small. Another reason for limited features is to prevent the app from becoming so complicated that it becomes difficult to use. In general, developers try to optimize apps to deliver a particular experience by playing to the strengths of a particular platform or operating system.
If Windows 8 were only meant for touch-screen devices, developers would only need to focus on how to create an app that works well with touch and gesture commands. But because Windows 8 also works on PCs that don't have touch screens, the mouse-and-keyboard users need to be considered during development, too.
What about software? While the word can mean any type of computer application, in general we now use the term to mean larger, more complex computer programs. These programs may have features that require more support than what the tiled interface can provide. They could require complex keystrokes or include deep menu systems that would be difficult or impossible to navigate in the tiled environment on Windows 8. These programs belong on the desktop.
The desktop environment of Windows 8 supports full programs. Some of these programs have app counterparts with fewer features or simplified interfaces. While the desktop looks more like previous versions of Windows, there are some major differences.
One of those is the lack of a Start button. Microsoft first introduced the Start button with Windows 95. For some users, its disappearance may be unsettling. You can find much of the functionality of the Start menu through the Start screen in the tiled interface. Clicking on a Windows button -- if your keyboard has one -- will switch between the desktop and tiled interfaces. Otherwise, moving your mouse cursor to the lower left corner of the screen brings up a Start screen icon.
Perhaps the most important role of the desktop environment is to act as a platform for older programs that have no app counterpart at all and which cannot run within the tiled interface. You may have a collection of legacy programs you depend upon regularly. The desktop environment helps ensure that Windows 8 is relevant to customers who need access to older programs.