The following is taken from the Google Chat back page; As you can see the information is clear and intuitive.
A Google Talk chat back badge will let visitors to your web page chat with you. They’ll be able to chat with you whenever you’re signed in to Google Talk [under your Google Account log-in] For more information, visit the help center.
Each time you visit this page, we generate a new badge for you so you can use badges with different settings on different web pages. For example, you can share your status message on your personal blog and not on your public blog. To change a badge later, just return to this page and get a new one.
You just build a chatback widget (eg badge) and then display it on your website. When you logged into Google it will automatically establish a Live chat system via Google chat back (eg messagener)
Google Chatback Demo
The below is a real live demo (where we are always offline!)
