Desktop Sharing is a server application that allows you to share your current session with a user on another machine, who can use a VNC client to view or even control the desktop.
You would typically use Desktop Sharing with the KDE VNC client, which is Remote Desktop Connection, since it closely matches the special features of Desktop Sharing.
Desktop Sharing doesn't require you to start a new X session - it can share the current session. This makes it very useful when you want someone to help you perform a task.
Please report any problems or feature requests to the KDE mailing lists or file a bug at http://bugs.kde.org.