From the tmux homepage:
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.
tmux is largely designed to be a differently designed and BSD licensed alternative to GNU screen. IRC support can be found in #tmux on the freenode network.
- Panes - similar to the ones found in Vim
- Client-Server model, allowing windows to be moved across sessions and allowing multiple client to share the same session
- 256 color support and UTF-8