One of the older players, and among the biggest Desktop environments, that comes with “everything and the kitchen sink”. KDE stands for the K desktop environment (Yes, Linux love Man in Black). It has been around since 1996, born to provide a unified look, feel and functionality to UNIX applications, which did not seem to have existed before, focusing on being easy to use.
Since 2009 KDE has re-branded itself from being a Desktop environment to be a suite of applications, growing even larger (which attracts quite a bit of criticism of being bloated). Now the Desktop Environment (most recently called the Plasma Desktop), and now includes hundreds of different applications, developed by the KDE team, that include everything from email clients to education software.
These applications all use the same framework (Nokia’s Qt), so they all look and behave in a similar way, making it super simple to adapt to, and a great candidate to learn the Linux desktop, providing consistency.
KDE has been thought of as the “prettiest” DE by its fans, while those who oppose love to call it “ugly”, with too much emphasis on eye-candy. This might or might not have been true in the past, but the PLasma 5 series introduced a really appealing design, that is a lot more subtle, and looking quite contemporary.
The previous major version of the DE is KDE (Plasma) 4. When first introduced, it has made many former users turn away from the DE, as it has seen a major design overhaul, redrawing almost everything, that made those who were used to KDE3 feel uncomfortable.
Although Plasma 5 is around for years now, Debian Stable still offers KDE4, so if you choose to go with KDE, you will likely use this version for a time to come. Update: This has been "fixed" since Debian 9 (Stretch) had become stable, and the upcoming Buster release offers a really cool new version of Plasma 5. Phew, KDE4's days are finally over.
Its functionality is very similar to what you might have been familiar with since Windows 95. It has a bottom panel, with an application launcher, task switcher, and a system tray, although the window decorations will differ significantly. And of course, the infamous blue “halo” around active windows (that has since been fortunately ditched) needs some getting used to
Another major version, and another major design overhaul, yet this time it was most welcome by the community, as it introduces some really eye-pleasing changes (like for example ditching the blue halo). This was the first major environment to adopt a flat design, and it did a fine job at that. Plasma 5 looks easy on the eyes and it’s easy to use while overwhelming you with functionality (which can even be a turn-off for some)