An ActiveX DLL is a DLL that supports OLE Component Object Model (COM). You can access these tools in web scripts written in other languages, like ASP. The DLL must implement and export the following procedures:

STDAPI DllRegisterServer(void);
STDAPI DllUnregisterServer(void);
STDAPI DllCanUnloadNow();
STDAPI DllGetClassObject(
REFCLSID rclsid, //CLSID for the class object
REFIID riid, //Reference to the identifier of the interface that communicates with the class object
LPVOID * ppv //Address of output variable that receives the interface pointer requested in riid );

Pre-installed on the Intermedia servers ActiveX components are listed in this article.