No, KDE is not a window manager. While KDE includes a very sophisticated window manager called KWM, KDE is much more than that--it is a full-blown Integrated Desktop Environment (IDE).
KDE provides a complete desktop environment, including a file manager, a window manager, a help system, a configuration system, uncountable tools and utilities, and an ever increasing number of applications, including but not limited to mail and news clients, drawing programs, a PostScript and a DVI viewer and so forth.