A desktop environment is a complete ecosystem of software and resources providing a homogeneous graphical user experience. Generally based on specific graphical widgets, configuration system, root window with desktop background, taskbar with window list and menu, icons, window manager etc. It provides a coherent visual metaphor for a user to interact with the computer.
A desktop environment can be a great convenience to many users, though a custom graphical environment can be assembled if preferred, i.e. Xorg could be used with a window manager and select desktop utilities. Gentoo's flexibility means that many desktop environments are available, and more than one can generally be installed in parallel. Gentoo can of course be used exclusively in text mode, as a server platform for example.
Gentoo does not have separate releases or “flavors” for different desktop environments, as some other distributions do - a desktop environment is simply installed on the base system.