GNOME 3.0 vs KDE 4.6
KDE 4.6 and GNOME 3.0 are competing desktops for GNU/Linux and other Unix-like systems. Both are free software.
Traditionally, KDE was the major Linux desktop. GNOME took this crown in the aftermath of the badly received KDE 4.0 release and became default in many popular distributions like Ubuntu and Fedora. The GNOME 3.0 release a few years later has brought KDE back as most popular desktop, with Ubuntu switching to their in-house developed Unity shell, openSUSE having KDE as default and the rising popularity of Cinnamon (a GNOME 3.x Shell fork) and Mate (a GNOME 2.x fork).
Shift in GNOME Support from Major Distributions
Despite their longstanding use of GNOME 2.x, Ubuntu has not committed to using GNOME shell (the most visible change coming out in GNOME 3); Ubuntu has chosen to create an alternative to the GNOME shell called Unity. Linux Mint 11 (Katya) will ship GNOME 3 (but without GNOME shell) as its default desktop environment - so its appearance will continue to be like previous versions of Linux Mint. Fedora includes GNOME 3 with GNOME shell.
KDE 4.6, meanwhile, has added features and polish to what was already a successful KDE 4.5 desktop. Community-supported editions of Fedora, Mint, and Ubuntu already provide a polished KDE4 experience. Prominent distributions that focus primarily on KDE as their interface include OpenSuSE.
Many of the free software world's most popular applications are neither designed for GNOME nor KDE, and work equally well on either. Software that is often used with KDE, GNOME, and other free software desktops includes OpenOffice/LibreOffice, Firefox, and Chromium.