Google replaces Google Talk with Hangouts, a cross-platform unified messaging service. With support for photos and emoji as well as video calls with up to ten friends, Hangouts is a required replacement for Talk.
Hangouts syncs across devices and only get notifications once. Interestingly enough, Hangouts doesn't support "invisible" status. The chat app also features photo sharing support, multiple accounts, and read notifications showing how far your contact has read in your chat.
Even if you use Google Talk, Hangouts will replace it with this update.