Difference between WLM and Windows Messenger

What is the difference between Windows Live Messenger and Windows Messenger?
I recently experienced some problems with .net Framework on my XP Home PC
and had to have .net Framework reinstalled. Windows Messenger version 4.7
came up as a default, and I couldn't find a way to check for updates. It
looks old, from 2004!

Then I downloaded Windows Live Messenger from microsoft.com, and got the
2009 version of it.

Now I have both Windows Messenger 4.7 and Windows Live Messenger 2009.
What's the difference? Do I need both? If so, why?

Thanks for the help...