Tmux cheatsheet in 21 lines

Sessions        # seperate instances of Tmux
Ctrl-b s        # Lists sessions in tmux
Ctrl-b $        # Rename current session
tmux ls         # Lists sessions on cmdline
tmux attach -t5 # Attach to session 5

Windows         # hidden windows, like tabs
Ctrl-b c        # Create new Window
Ctrl-b ,        # Rename current window
Ctrl-b w        # List windows
Ctrl-b l        # Open last window
Ctrl-b f        # find by name and cmd history

Panes           # split-screen windows
Ctrl-b "        # Create new Horizontal Pane
Ctrl-b o        # Switch active Pane  
Ctrl-b [space]  # Rearrage panes
Ctrl-b {        # Reorders panes
Ctrl-b z        # Zoom (full screen) active pane
Ctrl-B pageup   # Scroll back, ctrl-c to end

This page has a chart for screen users making the switch to Tmux