A desktop environment is a collection of disparate components that integrate together. They bundle these components to provide a common graphical user interface with elements such as icons, toolbars, wallpapers, and desktop widgets. Additionally, most desktop environments include a set of integrated applications and utilities.
Some seasoned Linux enthusiasts contend that there is no best desktop environment. Of course, like any piece of software, users will be swayed by personal preferences, the software they need, how well the environment integrates with the rest of the ecosystem, and so on. But there are still objective ways of assessing the environments, and the situations where they are best suited.