This is a quick overview of DDE – Dynamic Data Exchange.
Windows applications often require to exchange information with multiple application instances or even with different application instances. These applications may be running on a single machine or on several machines in a network environment. Several standard protocols have been developed to address these requirements. This communication is referred as interprocess communication.
DDE – Dynamic Data Exchange is among earlier versions of such data transfer protocols. It sends data to & from applications. This protocol can be used for one time data exchange or progressive data exchange. DDE was first introduced in 1987 & since it uses Windows Messaging Layer, therefore it still works in modern versions of Microsoft operating systems.
DDE – supports several Data Exchange formats & custom formats could be defined for specialized inter-process communications.
Sarfraz Ahmed's Blog 
Interesting one Sarfraz.