With many X GUI's, the minimize button (a little dot) will erase the window that the program is running in and create, instead, an icon on the desktop. KDE does not do this. Instead, when a window is iconified it is simply hidden (but the program is still running).
There are two ways to get the window back:
If you're running the taskbar part of kpanel (see the Panel --> Configure menu entry), you can choose to have a list of tasks displayed on you desktop. Iconified tasks have their names surrounded by parenthesis.
If you click the middle mouse button on the root window, kwm will give you a list of all available tasks (and again, the names of iconified tasks are bracketed).