ActiveX is Microsoft's marketing term for a set of software technology based on [COM].
When someone talks about ActiveX, they typically mean ActiveX controls
[http://www.microsoft.com/com/tech/ActiveX.asp], which are components
(that may or may not have a visible representation in a GUI) that can be embedded in an application.
To allow this embedding, ActiveX controls must implement specific COM interfaces. 
For example, a Visual Basic application may display a GUI containing widgets which are ActiveX controls.
Internet Explorer can download and run an ActiveX component in the browser window
(if you gave permission to install it).

Tcl extensions that support ActiveX in some way include [ActiveTcl] ([Tcl Dev Kit], both via [tcom]), [TclScript], [Optcl], [TclControl], [Noel Frankinet]'s [AxTcl] (well, not really ''his''),
[Joe Mistachkin]'s [TclBridge],
...

The [tcom] extension provides basic COM access, and does not specifically support the COM
interfaces for interacting with ActiveX controls.
That is, [Optcl] implements and accesses the COM interfaces required for
embedding ActiveX controls in Tk, while tcom does not.