Java Applets and web browsers
In the early days of Java it was assumed that its main area of success would be in Applets, small programs that get executed within the browser rather than on the web server. For various reasons applets have turned out to be a minority sport on the web. They suffered from performance and compatibility problems. Much of the compatibility issues stem from the different implementations from Microsoft, Netscape and Sun.
The Java Plugin
There are areas where nothing but an Applet will address the problem and the lack of compatibility across browsers was a thorny problem. As a result there was a minor industry in programming AWT applets that took account of the quirks between the Netscape/Microsoft/Sun implementations of Java. This ridiculous lack of standards was significantly addressed by Suns Java plugin. This is a kind of bridging technology that allows Applets to run using the standard Java runtime shipped by Sun in almost any browser.
The Java plugin gives a standard JVM target across all the major browsers.
The Plugin Tray icon
Under windows, when you access an applet the Java Plugin icon will appear in your system try in a similar way to the following screen shot (the arrow points to the icon)
The plugin will run in both the Netscape and Microsoft browsers and gives programmers a single platform as a target for their programs. I recommend the plugin as the preferred way to execute Applets as you can avoid many incompatibility problems between versions of Java.