A plug-in is one of a set of software modules that integrate into Web browsers to offer a range of interactive and multimedia capabilities.
Some plug-ins occupy a space on a web page, as in the case of certain MPEG
video
players. Those plug-ins may have a display area as well as video
controls for playing, pausing or rewinding, for example.
Other plug-ins, like some MIDI
players, needn't occupy any space on a web page. You might have encountered these plug-ins, which begin playing music as soon as you load a web page, but have no "on" or "off" buttons.
Many software engineers create their own plug-ins to enable software to run in a Web browser. Because software engineers may follow different conventions when designing plug-ins,
different plug-ins might behave differently among different browser types and versions.