VoiceXML is a nifty new technology that uses XML to enable voice interaction between wireless devices and middle-tier data services. Client-side technologies for wireless devices are a hotbed of debate and competition. One of the front-runners is WAP, which uses WML to define client-side interactivity. WML is an XML-based language that is analogous to HTML for WAP devices. VoiceXML is similar to WML in that it uses XML to define the user interface. It differs in that it uses voice instead of point-and-click. For a great introduction to VoiceXML, check out this article:

An Introduction to VoiceXML