An applet is a small application that runs inside another application, typically a browser. The bridge between the applet and the environment is provided by the Applet class which all applets must subclass. If the applet resides in a browser, it is downloaded over the network, checked by the bytecode verifier and executed if it doesn't contain any malicious code. These links are a very small subset of Applet information on the net. Check out any search engine for a great number of links.
In-house 
= answered, 
= unanswered