A plug-in (or an ActiveX control, called by Microsoft) helps the browser to accomplish specific tasks like playing specific multimedia file format as it has not got the support built in. For example, if web browser doesn’t know how to display a certain type of video file, it first looks for a plug-in capable of doing so but if plug-in is not found then web browser prompts to download it or searches for a application on the computer which can play it but, if the browser is unable to do so, then it won’t be able to display the file.
Plug-ins are usually free and can be easily downloaded from the Internet and sometimes, web browsers come with certain plug-ins. Sometimes, the plug-in installs itself and restart of web browser is needed.
Web browser gives options to display about installed plug-ins. For example, Firefox users can choose Tools | Add-Ons and then click the Plugins tab and for Internet Explorer, choose Internet Options from the Tools menu. Next, click the Programs tab, and then click the Manage Add-ons button to view a list of all add-ons (including ActiveX controls and plug-ins) used by Internet Explorer.
For example, Flash, a file type requires a plug-in, has become popular because they’re small and can include sound, video, interactivity, and animation. Also it’s plug-in is widely available on a large variety of platforms and browsers.
Earlier Microsoft used the <object> tag for ActiveX which works in Internet Explorer and other browser companies used the <embed> tag to embed items such as audio and video in a page instead. Hence usually the <embed> tag is in the <object> element for older browsers <embed> tag is not part of the HTML or XHTML specifications. In both the <object> and <embed> tag, the <param> tag is used to pass extra information to the player.