Wine; what is it?
Wine is a free implementation of the Windows API (Application programming interface); that is to say, it allows you to use Linux or Mac OS X to run softwares designed for Microsoft Windows. It is developed by WineHQ. This is the engine behind PlayOnLinux/PlayOnMac.
Wine uses what is called a WINEPREFIX to encapsulate Windows applications. This prefix contains the Wine configuration files and a reproduction of the file hierarchy of C:\ (the main disk on a Windows OS). In this reproduction of the C:\ drive, the installation of your software happens.
Under PlayOnLinux and PlayOnMac
The trouble is that Wine can be difficult to use and all applications do not work properly with this or that version of Wine. To avoid this inconvenience, PlayOnLinux/PlayOnMac will isolate the different versions of Wine, assign them to a specific WINEPREFIX, and allow the Windows applications to run with the Wine version most compatible. It additionally offers an uninstall option that is extremely simple; just delete the WINEPREFIX to completely uninstall Windows software.