tmux is a "terminal multiplexer", it enables a number of terminals (or windows) to be accessed and controlled from a single terminal. tmux is intended to be a simple, modern, BSD-licensed alternative to programs such as GNU screen.
Tmux (short for "Terminal Multiplexer") lets us launch multiple terminals in a flexible layout on a single screen, so that we can work with them side by side. For example, on one pane we can edit some config files with Vim, while on the other we are using irssi to chat and on other pane, tailing some logs. Then open another window to update your system, and another to SSH to some servers. Navigating through them is just as easy as creating all these windows and panes. It is perfectly configurable and customizable so it can become an extension of your mind.