Xfce is a lightweight desktop environment for unix-like operating systems. It aims to be fast and lightweight, while still being visually appealing and easy to use.
A priority of Xfce 4 is adhereance to standards, specifically those defined at freedesktop.org.
KDE or K Desktop Environment is a desktop environment for Unix/Linux built with TrollTech’s Qt GUI toolkit. With GNOME, it’s the one of the most popular desktop environments in use.
KDE as a project also encompasses a suite of applications to work natively under the desktop environment and stay consistent with the style and user interface of KDE, these include the suite of applications in KOffice and the integrated development environment KDevelop.
In the past, KDE has attracted some criticism for its use of the Qt toolkit, which was not released under open source licensing conditions, however it is now released under the GPL, so that GPL compatible applications may use it. (Non GPL compatible applications may not, such as MPL or proprietary licensed applications.)
In comparison to GNOME, KDE is considered by many to be closer to the user interface of Microsoft Windows, whereas GNOME is considered to be more unique or closer to OSX, depending on the configuration.