A month ago, Google has a number of communication services that simply do not talk to each other. There is Google Talk which is much like Yahoo Messenger, or AIM. There is Google Voice which uses Google Talk, but offers a phone number. This can also integrate with an Android phone. There is Google Plus Messenger which is a comparison to Facebook Messenger.
However, just recently (2013-05-21), Google has announce Hangout, and offers it as a replacement to Google Talk. It also offers video, and VOIP services for those using Google Plus. The idea will eventually be to replace all of the services with Hangout. Name aside, this is not a great idea, but they do have some parts in the right direction.