A desktop environment is a graphical layer between the user and the computer. In the UNIX context, a desktop manager is a graphical environment that runs inside an X session. It enables the user through mouse and keyboard interaction to access the underlying features of the operating systems. A full desktop environment (as opposed to a window manager) usually provides a set of software for the most common needs (file access, web browsing, printing…).
A desktop environment usually consists of many of the following components:
In addition, Desktop Environments usually provide integration with features such as display and ACPI power management, useful for notebooks/laptops. Modern Desktop Environments also provide or integrate with a desktop compositing engine which allows for graphical special effects such as 3D flipping of workspaces, window translucency, simple animations and shadows, but this requires 3D hardware acceleration enabled and can consume additional system resources.
Desktop environments available in Slackware can be classed as full or light-weight environments depending on the number of features they include. The more features an environment has, the more complete it is but also the more computer resources it uses. Light-weight environments come with a smaller set of features, usually just a unified look and feel, so you will need independent applications. For example a video player is not included like it is in KDE, but you can use xmms which is installed by default with Slackware.
Switching between available environments is done by running xwmconfig from a terminal prior to starting X.