My gratitude to Chris Bailey in the Internet Development group of ILRT for his permission to pass on the PowerPoint slides from a short (six-minute) talk he gave this afternoon.
XForms is a higher-level language for describing the interactions done by web forms. It abstracts the data model from the presentation, gets you away from coding the form inputs and behaviours individually, and makes data validation a whole lot easier. Presently, XForms is not supported natively by browsers, so either you need to do server-side conversion or the user needs a suitable plug-in.
In a remarkable forward-looking move, XForms are recommended by the UK Government’s e-government interoperability framework (pdf link).
Wikibooks has an XForms Tutorial and Cookbook which shows, amongst other things, how XForms can be used to make simple applications such as a calculator.