I came across a java applet on a webpage and I wanted to know how it worked. These .class files are not human readable if you open them in a text editor like notepad.
When you view the source of the webpage (IE just right click on the page and there is "View source" option) you can find what .class is running, which is the underlying program for the applet.
After that you can use something like JD-GUI, a java decompiler, to view the code