The K Desktop Environment® (KDE®) is a graphical desktop and a set of applications. It is created by a world-wide community of people dedicated to creating a free and user-friendly computing experience.
The KDE desktop runs on various free operating systems, such as Linux and UNIX. There are various ways of trying KDE. One way is Live CDs that make it possible to try KDE out on your computer without touching your valued data. There are also images for various virtual machines, such as VMware and QEmu. The Try KDE! page lists some of those. You can use KDE free of charge; there are no licensing costs.
You can also choose to install and run one of the Linux distributions that ship KDE.