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 Sourceforge project page of tmux.