Away is when you change to status to... 'away'
Idle is when you have the status 'online' and go do some other stuff (doing the laundry, watching tv)
after some while, WLM switches to 'Idle' becuase you are inactive...
So 'away' you choose yourself
and 'idle' is when you're not active...
So if anyone of your contacts has the status idle, it most likely is they didn't use WLM for a while...
I have the dutch version, so I don't know exactly how it's called, but at the same page you can change your name in WLM, there's also an option "change status to 'away' after (...) minutes", WLM names it 'Away' but this is a different 'away' than when you change your status.
MP!L sees the 2 different away's and seperates them as 'away' and 'idle'
-edit- Lol
, both got beaten by Zahid