Google+ Messenger (formerly Google+ Huddle) is a part of the Google+ app, which is more meant to be a SMS-like service. Your conversations is always available on Google's servers, even though you decide to uninstall, and later reinstall the Google+ app again.
You can also share photos, and other content, through the Google+ Messenger service, to your other Google+ circlings.
When you receive a Message and / or incoming Hangout, then your smartphone will ring and vibrate just as normal calls and texts.
Google Talk has been around for many years now, and is the Instant Messenger (IM) service from Google, which is available for all platforms, both in the browser, and by installed software.
Both services support Hangouts, which is the webcam and / or voice calling part of Google.
Those Google Hangouts can both be public, for a specific circle, or private.
Google Hangouts has recently been very popular among many major TV-stations, celebrities (i.e. President Obama), organisations, companies, and others, where "normal" people can come and join the conversation.
A normal Hangout supports upto 9 participators at the same time, and some are streamed Live on Google+ and YouTube.
I hope that helped you understanding the differences, between Google+ Messenger and Google Talk.