What is a Mecanim Avatar?

Unity 4.0 is first experience scripting animations in Unity, and so far I'm impressed.

I can't find any documentation that really explains what this Avatar thing is.

It's clear to me that I need to provide it with the joints it requires, and the less it complains the better.

It's also clear to me that I can have joints that the Avatar doesn't know about and they still animate.

Is the primary purpose of this thing just sharing animations between characters with different skeletons?

Here's an example of the type of thing that confuses me...

If you look at the robot character provided in the Unity 4.0 Mecanim tutorial (http://video.unity3d.com/video/7362044/unity-40-mecanim-animation) and look at his avatar setup, his Root joint is mapped to the Avatar's hips. If you try to set the Avatar's hips to be the robot's hips it complains because the skeleton hierarchy doesn't match what it's expecting. Basically his hip joint isn't mapped to anything in the Avatar.

All of the animations you play on the robot are targeted at the Dude Avatar. The Dude avatar has all it's joints mapped correctly.

Why do the robots hips animate correctly? I'm playing animations targeted at the Dude's Avatar on it, shouldn't the Dude's hips be driving the robots root?

TL;DR can someone explain to me what this Avatar thing actually is and what it does?

Thanks!