Using Google Hangouts and chat
Get the App
Before you get started, you will need to download the Google Hangout plugin. It's free and easy, and loads within seconds: https://tools.google.com/dlpage/hangoutplugin
Note: Plugins no longer work in Firefox web browser's latest version 52.
Start a Google Hangout
To access and start a video call, phone call, or chat session, visit: https://hangouts.google.com/
To start just a video Meeting, visit: https://meet.google.com/
Add a Hangout to a Google Calendar Event
You can add a Hangout to any Google Calendar event by clicking on the Add video meeting details link under the Where location on the event details page. The people who are invited to the meeting will have the option to Join the meeting via Hangout by clicking on the meet.google link in their Google Calendar.
More information on managing a Google Hangout meeting.
Google Hangout help center: https://support.google.com/hangouts/?hl=en#topic=6386410
A similar function for a quick online chat or to launch Google Hangout is a gmail chat.
Here's a handy guide to setting that up so it is available from your gmail inbox screen.