A DE is a integrated desktop - usually has a common look/feel for all the applications that use it, etc. Only ones I really can name off the top of my head are KDE and Gnome. A window manager does just that- manage windows. Gives them a border, the close/minimize/maximize buttons, scroll bars, etc. Technically you don't need either to run X apps, but if you just use the X server there will be no easy way of starting an app (no menus, etc.) and no way to control their position.