In MicrosoftWindows, you can drag objects (text, graphics, etc) from one window to another by selecting them, dragging it with your mouse, and dropping it into the new window.
XFree86 does this a bit differently. Drag and drop is a two stage process. You select the item you want to move. You don't physically have to drag the item, you just move the mouse to where you want to go, and then you drop using the middle mouse button. 1?2?
This uses the Primary selection buffer. See http://www.jwz.org/doc/x-cut-and-paste.html for the rationale.
This is not CutAndPaste.
See also the XDND, the Drag and Drop Protocol for the X Window System.