Here's what you'll need to get started:
Download the voice and video chat plug-in, quit all open browser windows, and install the plug-in.
Sign in to Gmail, iGoogle or orkut.
In your chat list, select the contact you want to call. If they have a camera icon next to their name, you can make a voice or video call to them; just click Video & more.
If your friend doesn't have a camera next to their name in your chat list, you can invite them to download the voice and video chat plug-in from the Video & more menu in a chat window. Even if your friend doesn't have a video camera, you can still have a voice chat or a 1-way video chat.