There are basically three kinds of plugins available with web browsers. These are:
Microsoft <OBJECT> tags enclosing ActiveX objects
Netscape <EMBED> tags enclosing browser plugins
Java <APPLET> tags enclosing Java applets
The distinctions between these different plugin architectures become increasingly blurred and basically all of their functionality could probably have been provided with a single mechanism had the browser manufacturers worked together more co-operatively and been less concerned with carving out territory and gaining market share at each other's expense.