These are commonly misused terms in Internet communications:
Video Conferencing - provides real-time two-way audio/video communication between two or more locations. Video Conferencing requires specialized equipment on both ends for a successful connection.
Web Conferencing - provides live audio/video communication between two or more locations to conduct meetings, training, or presentations via the Internet. In a web conference, it is common for each participant to sit at their own computer and connect to the web conference via the Internet and a phone or VOIP (which uses the Internet for real time audio communication and eliminates the need for a telephone).
Webinar - a specific type of web conference that is typically one-way (speaker to the remote audience with limited remote audience interaction). There can also be a live audience in the room with the presenter of the webinar. Polling participation may be integrated into webinars to allow for some remote audience participation.
Webcast - is essentially "broadcasting" over the internet or streaming. In webcasting, an audio and/or visual content source is distributed simultaneously to multiple viewers/listeners. This content is usually distributed live and is usually non-interactive.