TerminalMUltipleXer it's called and that's what it does.
With this little program you are able to log into a Remote system and let sessions open for later use. For Screen you have to extensively tweak it to do your belongings and tmux just has it built in. My killer feature for this app is it's ability to split Terminal Windows just like a tiling window manager does.
Let's say you have 200 ssh sessions open each in one seperate window.
Now search through the windows with screen:
you get it.
Search for a specific text in all terminal windows in tmux:
ctrl-b f myservernumber1
you got a list of all windows with that specific string in it.
I still not discovered all features but it's insane what it does to your ssh and console behavoir.
Go and try it out.
apt-get install tmux