Each session has a name associated with it that can be used when attaching to them. Session names are displayed when listing your sessions using screen -list.
root@backup:~# screen -list There are screens on: 7637.dispatch (Attached) 7545.playing (Detached) 7680.irc (Detached) 7684.logs (Attached) 7693.pts-5.backup (Attached) 4 Sockets in /var/run/screen/S-root.
The default session name is constructed from the tty and host names, which isn't very useful as you can see from the last entry above. To give a screen a name, you specify it with the -S option.
screen -S mySessionName
It's even more useful when combined with -R, in which case the -S can be omitted. This causes screen to attach to an existing session with the given session name, or create a new one with that name. Combined with -d or -x, it's a very powerful feature. :)
screen -xR mySessionName
Session names can be changed on an existing session using the :sessionname command.
C-a :sessionname mySessionName
Session names, combined with window title_examples, makes life a lot easier when you have multiple sessions with multiple windows in each.