An Introduction to the KDE Desktop
There are two big players in the Linux desktop GUI arena: Gnome and KDE. Ubuntu, Fedora and Mint all supply a KDE alternative (-known as Kubuntu, Fedora KDE Spin and Mint KDE respectively) to their default Gnome distributions.
Both Ubuntu and Fedora supply a KDE alternative [..] to their default Gnome distributions
Both KDE and Gnome provide similar functionality -and each one has fiercely loyal advocates. However, with the introduction of Gnome 3.x (-and Unity) - which introduced a radically different way of working (-essentially dispensing with drop-down menus) - a lot of Gnome users have become disaffected: either because of the lack of backward compatibility/application compatibility or due to the very different method of interaction with the desktop.
KDE is often referred to as the "power user's desktop", because it is much more configurable / customizable than Gnome. However, you may find the learning curve for KDE a little steeper -at least, should you wish to explore the myriad of options available.
Of the two, KDE is the more resource-hungry: this may be important if your hardware is old or disc space is tight.
If you are new to Linux, then which desktop you choose will depend purely on personal taste: both do the job brilliantly!
Like any evolving software, KDE issues periodic upgrades and releases. The latest KDE desktop is known as plasma (-version 4.8.0). When looking for your ideal Linux distro, be sure to check which version of KDE it ships with.
As long as you download the respective Gnome libraries first, you can run Gnome-based software under KDE or vice versa. Running KDE-based software under a KDE desktop means only that the application is likely to have the same look and feel with the GUI - making it a little more intuitive to use for experienced KDE users.