I cant find any reference which explains how Add-ins, Plug-ins and Extensions differ from one other. I just cant accept them to be synonyms.

Here is an account of how I interpret each one. Please correct me or point me to appropriate source.

Add-ins
I guess add-ins are those modules which bring-in a specific fuctionality, at runtime, for such requirements which are unknown at build time. That mean Application is unaware about source of Add-ins. Normally they are chosen from list to add. eg.Excel Ad-Ins

Plug-ins
I guess Plug-ins are those modules for which an adapter is built inside the application at build time. Thus a trust relation ship exists between application and Plug-In at Runtime.

Extensions
I guess Extensions are those modules which are added to bring in additio ...

Last week some one tried to make a joke about usage of Net DDE in WW products. Actually I was also not so convinced why WW still supports DDE and SuiteLink (DDE+ avatar). Since then I was trying to collect some information to educate myself on DDE.

I have been using DDE since 1998. Then I used it to create some custom report formats for customers. I used to collect information from DCS Systems to populate excel sheets. Those were the times OPC (OLE for Process Control) is stabilizing. OPC is COM based as the name itself contains OLE. Every DCS system is offered with DDE support for connectivity to excel. A simple formula in excel to access data made is quite a suitable approach. Like Application!Topic.Item for local DDE. and \\server\ndde$|share_name!tag_name for Net DDE. This is simplified by WW to be \\ ...