Drag and Drop is an interactive technique which allows transfer of information through a simple metaphor: the user picks an object on the display with the pointer, drags it across the screen while maintaining the pointer button pressed, and drops the object on some other place. Subsequently, the application where the object was dropped usually does some data transfer corresponding to the object.
A Drag and Drop operation starts in a window called the source window (or simply drag source). The client owner of this window is called the initiator client. The drop operation occurs in a window called the destination window (or drop target). The client owner of this window is called the receiver client. A client can be both initiator and receiver for an operation. A client can contain both the source and destination windows for an operation. The source window may also be the destination window. There are no particular requirements on what the client should do or not do in those specific cases.
In many cases, the drop will occur on some area called a drop site, for example a printer icon or a trash can icon. A drop site may be drawn in its own window or may be simply drawn by the receiver application in a larger window. The protocol does not require to have one window per drop site.
During the drag, both the initiator client and the potential receiver clients may want to provide visual feed back to the user. The feed back provided by the initiator is called drag-over. The feed back provided by a receiver is called drag-under: typically doing some animation in the destination window under the drag icon.The feedback may consist in:
Part from an article for the Motif Drag & Drop Protocol written by Daniel Dardailler.