D-Bus is the message bus used in KDE 4, that is used for inter-process communication (IPC) between different KDE applications. D-Bus can be used in order to remotely control KDE applications from your own application or to provide scriptable interfaces for your own application. It has replaced DCOP in the KDE 4 release.
KDE is network transparent. This means that each application can access files via many network protocols just as if they were stored locally. There exist access modules (KIO-Slaves) for Samba (Windows Shares), HTTP, FTP and many other protocols.
KDE actively supports The Portland Project aiming at making cross-desktop application development easier. For more information on this fairly young project, please refer to The Portland Project's website hosted on Freedesktop.org.