What's the difference between a screen being attached and detached?

Question

[blah]$ screen -ls
There are screens on:
        16517.pts-0.blah       (01/28/2012 01:55:32 PM)        (Detached)
        4345.pts-1.blah     (01/27/2012 11:22:48 AM)        (Attached)
2 Sockets in /var/run/screen/S-blahblah.

Got 2 questions

1) What's the difference between attached and detached screen

2) Can you kill attached and detached screen and how do you do it? (I killed attached screen with screen -X -S id kill before but I don't think it works on detached one)

Answer 1

Attached means some process is actually using that session, detached means someone left the screen session running, but disconnected from it.

Why not simply connect to the screen sessions one at a time, and simply stop everything that is running? Connect to existing sessions with the command screen -r -d, or screen -R -D.