Basically it is a small program that is used to share information between applications. An ActiveX control can enhance Internet browsing experience by allowing animation and is often compared to Java applets. Microsoft developed the concept in the mid 1990s.
While technically still around technology is no longer commonly used in modern web development.
An ActiveX control is similar to a Java applet. Unlike Java applets, however, have full access to the Windows operating system. This gives them much more power than Java applets, but with this power comes a certain risk that the applet may damage software or data on your machine.
To control this risk, Microsoft developed a registration system so that browsers can identify and authenticate it before downloading it. Another difference is that Java applets can be written to run on all platforms, whereas ActiveX controls are currently limited to Windows environments.