What is the differrences between Talk and Google+ Messenger?

Both are the chatting tools of Google. I use Talk since I use Gmail to chat with my friend and when I see Google+ Messenger, I just thought it is Talk but the two are NOT the same at all - i.e. chatting with one person on Talk and he/she will NOT receive it Google+ Messenger. This is so confusing. What is the differences then? Why there are two chat tools like this? *Update* The funny thing is that Google+ Messenger is used for chatting with Google+ users from Android phone. But when I log in my Google+ page, I can ONLY talk with Talk friends !?!? I want to continue my last chat with a friend on Google+ from my PC, but now I cannot. So funny!