Google released Hangouts last week as a standalone app for iOS, Android, and the desktop via a Chrome extension. In addition to the apps, you can also use Hangouts from within Gmail, just like the old Google Talk.
To enable Hangouts in Gmail, click on your profile photo from the Google Talk chat list, then select Try the new Hangouts. Hangouts will immediately replace Talk, then walk you through a very brief introduction to Hangouts.
Keep in mind that it may not have rolled out to you yet, so if you don't have the Hangouts option in your Gmail account, try checking again later. Also, if you decide you don't like Hangouts in Gmail, you can switch back. Just click on your chat profile photo again, then choose Revert to old chat.