Gmail calling works like a regular phone, except that you need to have your e-mail account open. To call someone, click on the Call phone option at the top of your Gmail chat list and dial a number or enter a contact's name. In addition to voice, Gmail calling allows you to video chat with supported webcams and cell phones. Gmail calling also does international phone calls too, starting at $0.02 a minute.
Calling in Gmail is currently only available to US based Gmail users. You can start using it on Windows, Mac, or Linux by navigating to Google.com/Chat/Voice.