"A VoiceThread is an online media album that can hold essentially any type of media (images, documents and videos) and allows people to make comments in 5 different ways - using voice (with a microphone or telephone), text, audio file, or video (with a webcam) - and share them with anyone they wish. A VoiceThread allows group conversations to be collected and shared in one place, from anywhere in the world."
http://voicethread.com/
The Voicethread 4 Education wiki, started by Colette Cassinelli, is a comprehensive collection of VoiceThread examples from students and teachers of all ages and groups. The purpose of this wiki is to gather examples of how educators are using Voicethread in their classrooms (or for professional development) and to share those examples.
http://voicethread4education.wikispaces.com/