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KDE Plasma 5
I've found KDE Plasma 5 to be very fast and strong.
KDE also has its own application family. For many years, the apps and the desktop were tied at the hip. You could use other programs with a KDE-based Linux desktop, but at times it wasn't easy. Today, the two are separate.
The other major Linux desktop interface, GNOME, started as a reaction against KDE. Over the years, KDE and GNOME supporters have clashed frequently, but the hostility between the communities has declined over the years.
One reason that conflict cooled down is that GNOME 3 lost a lot of its popularity. People, including yours truly, really hated GNOME 3 when it first came out. Linus Torvalds, Mr. Linux himself, disliked it so much that he called for a GNOME fork in 2011.
That's exactly what happened. There are two major GNOME forks, Cinnamon and MATE, which are covered later in this overview.
GNOME, now up to version 3.12, has corrected many of the problems that early adopters had with it, and has been gaining back some of its fans. You can try it out on Arch Linux, Fedora, or Ubuntu GNOME.