Dynamic Data Exchange
Dynamic data exchange (DDE) is an old technology for communication between multiple applications under Microsoft Windows and also OS/2. Although still supported in even latest Windows versions, it has mostly been replaced by its much more powerful successors OLE, COM, and OLE Automation. However, it is still used in several places inside Windows, e.g. for Shell file associations.
In particular, DDE allowed one application to open a session with another, send commands to the server application and receive responses. However, it did not enable the server application's GUI to be incorporated into the client application's; it did not support incorporation of server application data into client application files (i.e. structured storage); and in order to use DDE, one had to know the DDE commands that the server supported, which were generally not standardised (although some standards do exist, e.g. the spyglass specification for web browsers). Thus, for full use of DDE to be used, special code must be added to each client application for each server application it wants to control, or the client application must provide a scripting language or macro facility.
A common use of DDE was for custom developed applications to control off-the-shelf software, e.g. a custom inhouse application written in C or some other language might use DDE to open a Microsoft Excel spreadsheet and fill it with data, by opening a DDE conversation with Excel and sending it DDE commands. Today, however, one would instead use the Excel object model with COM or OLE Automation.
Windows has a facility called NetDDE, which enables DDE messages to be sent between applications running on different machines. It is even more rarely used than DDE, but still supported. Notable applications using NetDDE are Clipbook Viewer and Microsoft Hearts.