When dealing with java applets it is important to remember that they are executed based upon a collection of permissions that prevent the applet from performing certain tasks unless permission is granted.
An unsigned applet has a collection of strict, almost draconian, restrictions, but for good reason. These restrictions are in place to protect the user from the actions of applets of unknown source. Unsigned applets are not allowed to access certain system properties or create files on the client machine among other things. It should also be noted that the applet can only connect to the server it originated from.
A signed applet is able to run almost completely as a native standalone application. They can write files on the client machine, access system properties and more.