What is KDE?
KDE is created by a group of people producing software oriented toward a great user experience. The software includes a set of libraries, an application collection (games and educational applications, web browsers, office and creativity suites, PIM applications such as email, addressbook, and calendar), and a set of workspace shells (targeting desktops & laptops, netbooks, or tablets and touch-screen devices (the latter of which is not yet packaged for Debian).
KDE in Debian
You can find information about KDE's software in Debian from the Debian Qt/KDE maintainers website.
DebianSqueeze contains KDE's 4.4.5 release.
DebianWheezy contains KDE 4.8.4 with KDE PIM 4.4.11.
DebianJessie contains KDE 4.11.3.
DebianUnstable contains KDE 4.11.3.
There are three options to install KDE Plasma Desktop in Debian:
How to install
"KDE Plasma Desktop"
Debian's selection of applications
KDE (Full release of workspace, applications and framework)
The standard/upstream release
KDE (Debian selected common stuff for workspace, applications and framework)
A debian selection of common things
KDE Plasma Desktop
This is a minimalist plasma desktop
KDE Plasma Netbook
This is a minimalist plasma netbook
Watch out recommended packages (i.e packages dependencies) !
(you might or might not want to install them).
Installing "KDE Desktop" task
KDE Desktop task is what is installed by Debian-Installer's Desktop "task" (if you typed install desktop=kde at DebianInstaller CD prompt.)
apt-get install aptitude tasksel
Then, install the kde-desktop task:
aptitude --without-recommends install ~t^standard$ ~t^desktop$ ~t^kde-desktop$
(This is what DebianInstaller would have installed in a new system, but you might want/need to uninstall the current display manager).
KDE Desktop - Content
#tasksel -t install kde-desktop aptitude -q --without-recommends -o APT::Install-Recommends=no -y install ~t^desktop$ ~t^gkde-desktop$
As of DebianSqueeze, this corresponds to:
(tasksel --task-packages desktop ; tasksel --task-packages kde-desktop) | sort -u
The appearance of GTK-applications in KDE
To get gtk-applications look native in KDE you need install these packages and configure appearance of them through System Settings > Application appearance.
KDE configuration module for GTK+ 2/3.x style selection
Oxygen widget theme for GTK2+-based applications
Oxygen widget theme for GTK3+-based applications (Debian Wheezy)