Google stated that their Google Talk will be replaced by Google Hangouts. Like Gtalk Hangouts is also (partly) based on XMPP which is a free and very well tested standard.
As a first real consequence of that statement the Google Talk app for Windows has been deprecated as of February 23, 2015.
But the GTalk services still working and we have mid of March 2015 now. Why?
Some people wrote about that Google just has stopped the federation services which means the communication over XMPP with others servers like jabber.org etc. That meant you are still able to use GTalk but for communicating to other GTalk users only.
But well it may be true but that leaving the fact that other servers has stopped their communication with Google instead. Why? Google is not willing to support encrypted standards which are a requirement since a while in the protocol. As a consequence using GTalk will be a problem when you expect to talk to users who are not using Gtalk as well.
I highly recommend to switch over to another XMPP based service which is respecting security and open standards.
For the full details and all Google statements take a look here: xmpp.org (link)