ActiveX is a broad term that generally refers to a Microsoft binary standard for sharing code and data between separate programs or "components". The actual protocol is specified by the Component Object Model (COM). COM has undergone many upgrades and enhancements since its original introduction as Object Linking and Embedding (OLE).
One important feature of COM is Automation. Automation allows a "client" program to control a "server" or component program just as if the component functionality was built into the client. For example, DADiSP can fully control Microsoft Word, placing a DADiSP Worksheet into a document and printing the results all from within DADiSP. Likewise, a Visual Basic program can hand off data to DADiSP for processing without DADiSP ever being visible to the user. In this way, DADiSP acts as a powerful data analysis library for any program that supports Automation.
ActiveX components generally expose three basic constructs:
Properties represent the various settings of the component such as color, font, size, etc.
Events are notifications the component makes to the client program. Some possible events are mouse was clicked, canvas was sized, button was pushed, etc. After receipt of the notification, the client can respond appropriately. In some cases, the client program can also fire events to the server.
Methods are functions the client program can direct the component to execute.