In graphical computers, a desktop environment is used to provide a graphical user interface (GUI). Different from the command line interface (CUI) of the early age, a typical desktop environment provide software components like icons, views, toolbars, folders and wallpapers, along with multimedia programs, system widgets, games, utilities, develop tools and many other things. In a word, the abounded features in designing and functionality of desktop environments bring users a quite different looks and feels.
The open source Linux has many desktop environments that can be replaced with each other; while the popular closed source operating systems like Microsoft Windows and Mac OS X use its own dekstop environment which are conservative, though some themes and third-part softwares can be used to change looks of the interface (such as windows, buttons and icons). In Windows, all changes can be done by replacing default theme in Explorer shell.
A desktop environment consists of software packages, and different desktop environment may have quite different components, which can diverge in version as well. Because of the dependency problems in Linux system, not all desktop environment can be installed at a same time.