Google Talk is browser plugin that allows you to make a voice and video chat with your friends via your webcam and microphone connected to the computer.
With the plugin installed, just log in to your GMail account with Firefox or Chrome browser, select a contact, you will be able to see that you can have a video chat (you get the option for video chat if they have a camera, or the video chat icon is grayed).
Install Google Talk Plugin:
Press Ctrl+Alt+T on keyboard to open the terminal. When it opens, run the command below to add the Google Talk repository:
sudo sh -c 'echo "deb http://dl.google.com/linux/talkplugin/deb/ stable main" >> /etc/apt/sources.list.d/google-talkplugin.list'
Then get the key:
wget -q -O - https://dl-ssl.google.com/linux/linux_signing_key.pub | sudo apt-key add -
Finally update your system and install the plugin:
sudo apt-get update sudo apt-get install google-talkplugin
Once installed, Firefox users can verify that if the Google Talk plugin is installed by going to Tools->Addons->Plugins. Chrome users go to chrome://plugins/