If you remember Applets, then you are so 1990’s man. Right back before the dot-com boom , everybody was putting these Java programs in their web pages to do things simple things like display a financial chart (guilty as charged , my ‘lud). Oracle still uses them in some versions of it’s applications as a half-way house between it’s older desktop applications and a completely web only solution.
Fast forward to 2006 and these ‘heavy’ applets have been replaced by light web pages using Ajax and Flash, which don’t require a user download to run. Pretty much anything a Java Applet can do can be done in Ajax (if you have enough time and patience). Ajax developers have been pretty inventive in using Flash to solve problems (e.g. allowing web pages to store information on your local PC), so it may only be a matter of time before this gets picked up.
Jan of Trampoline Systems explains in more detail.