Decker is a "multimedia sketchpad". It's a drawing program, a database, and a rapid prototyping environment. A "deck" can be exported as a self-contained, self-executing .html file (as seen above), and there's also a non-browser-based native application. It uses a functional scripting language named Lil which incorporates ideas from the APL family. You can learn more and play with some additional examples here:
Aside from the scripting language, a major innovation Decker has over HyperCard is the concept of "Contraptions": user-defined interactive widgets that can be reused by simply copying and pasting them between decks. If you can make a card, you can make a contraption, and then you can share it as a new building block:
http://beyondloom.com/decker/