David,
try googling a bit.
I'd not suggest _starting_ with DDE at this point, unless really necessary.
It is a rather old technology, if not to say nearly obsolete.
DDE means dynamic data exchange and was introduced several (>10?) years ago, maybe in Win3.1x or W95.
It still works with several SW, but support might fade away sooner or later.
In order to perform DDE-based actions you need to know the DDE commands and syntax of the SW you need to interact with.
Greetings from Germany!<br>-- <br>Uwe