tmux is a terminal multiplexer: it enables a number of terminals (or windows), each running a separate program, to be created, accessed, and controlled from a single screen. tmux may be detached from a screen and continue running in the background, then later reattached.

Features that differentiate tmux from GNU Screen are:

  • menus for interactive selection of running sessions, windows or clients
  • window can be linked to an arbitrary number of sessions
  • vi-like or Emacs command mode (with auto completion) for managing tmux
  • lack of built-in serial and telnet clients
  • different configuration language
  • different command keys—it is not a drop-in replacement for screen

For more information have a look at the project page of tmux.