Terminal multiplexers can manage multiple shell instances, or "windows," at the same time within a single session. They allow the user to detach and reattach terminal sessions, leaving any shells and their subprocesses managed by the multiplexer while detached. And more than one user at a time can simultaneously connect to the same session.
A comparison page
http://www.wikivs.com/wiki/screen_vs_tmux
Other links
http://www.techrepublic.com/blog/linux-and-open-source/is-tmux-the-gnu-screen-killer/1901/
http://dominik.honnef.co/posts/2010/10/why_you_should_try_tmux_instead_of_screen/