What are the desktop environments that use less resources on Linux ?

One of the main features of Linux is to provide a complete and functional operating system that can completely work also on dated PC where Windows XP has now become too old and the new Windows 7 requires too many resources.
Linux can revive pc set aside because the hardware is no longer supported by Microsoft operating system and so providing a fast, complete and secure environment.
On GNU/Linux there are many desktop environments some specially designed to be lightweight but at the same time they offer customization, effects etc..
In this article we will see some of the ???light??? desktop environments for Linux, the consumption of resources required and how to install them.