Video calls:

•     Are a one-on-one video and voice conversation with a single person at a time.
•     Requires a connected webcam, microphone, and speakers.
•     Requires that you and your friend use Yahoo! Messenger 10 or higher.

Webcam sharing:

•     Is a live video feed that can be broadcast to one or more people at the same time (who are able to access it only with your permission)
•     Requires that you launch a separate PC-to-PC voice call if you want to have a voice conversation at the same time.
•     Only requires a connected webcam.
•     Allows you and your friend to be on different versions of Yahoo! Messenger.