Webcasts

"A webcast is a media presentation distributed over the Internet using streaming media technology to distribute a single content source to many simultaneous listeners/viewers. A webcast may either be distributed live or on demand. Essentially, webcasting is “broadcasting” over the Internet." (1)

They are usually interactive, allowing viewers to participate using a chat facility. To participate in a webcast is generally free, but there may be a registration process. The only equipment required is a networked computer with a soundcard. After the live broadcast, webcasts are often archived so that they can be accessed at any time. 

Examples of webcasts:

References
1. Webcast [Online] Available from: http://en.wikipedia.org/wiki/Webcasting [Accessed 5th October 2011]