What is tmux?
tmux is text mode multiplexer – connection and session manager making things much easier if you're working with more than a server or two at a time.
What tmux does
tmux manages your text sessions using windows and panes in them, allowing you to quickly switch between sessions using hotkeys (meta-key plus tmux command).
Most useful benefit of tmux is that it keeps your sessions running even if you disconnect from the server tmux is running on. So when you're doing a remote task that's taking long time – it's best to start it in a tmux session, because you can disconnect from server but tmux will keep session running and your task executing. You can reconnect to the server and join the same tmux session as if you never left it, continuing with your task.
How To Use tmux
How To Configure tmux
Most of configuration is done using .tmux.conf file in your home directory. You need to restart tmux after updating this configuration or invoke a tmux config file reload.