A Brief History Lesson
The original web browsers were very simple affairs. They could display images and black and white text but bells and whistles like multimedia files or even colored text were out of the question. Netscape changed that. They realized there was no way they could build support for every possible type of file into the browser so they devised a method that matches the extension and type of a file with a "helper" application that was hopefully already installed on the users computer. Netscape implemented this by adding the embed tag and a special kind of application called a "plug-in" that allowed other pieces of software to display files directly within the browser.
The World Wide Web Consortium (W3C), the international body responsible for setting standards for the Internet, chose to ignore the successful embed tag in favor of its own object tag. Microsoft, never one to pass up a chance to get a leg up on its competitors, seized on the object tag as a way to increase their influence in the browser market. Also in typical Microsoft fashion, the company implemented the object tag in its own non-standard way using their private ActiveX technology. Netscape and Firefox now fully support the object tag and, for older websites, the embed tag. Microsoft, on the other hand, dropped support for the embed tag in Internet Explorer 5.5.
A plug-in is a special application that allows another piece of software, e.g. a video player, to run inside the browser. Let's say you have an audio file on your site called "mymusic.mp3" and you add a standard hyperlink like this" <a href="mymusic.mp3">Play my song</a>
If a user clicked on this link, an audio player like Windows Media Player or Real Player would open up and start playing your song. The specific player would be the one set up as the default player for mp3 files on the user's computer.
If you want the external file (i.e. the audio or video clip) to play directly in the browser then the user has to have the correct plug-in installed. A plug-in is like a middleman that lets the player software and the browser work together to display a file.
It is possible that a user won't have the right plug-in for a file that appears on your page. In that case, the browser should prompt the user to automatically download and install the plug-in but you have to tell it where the plug-in can be found. We will see how to do this in the next section.