start a new screen session: screen
.. with session name: screen -S
attach to a running session: screen -r
.. to session with name: screen -r
the “ultimate attach”: screen -dRR (Attaches to a screen session. If the session is attached elsewhere, detaches that other display. If no session exists, creates one. If multiple sessions exist, uses the first one.)

All screen commands are prefixed by an escape key, by default C-a (that's Control-a, sometimes written ^A). To send a literal C-a to the programs in screen, use C-a a.

detach: C-a d
detach and logout (quick exit): C-a D D
exit screen: exit all of the programs in screen.
force-exit screen: C-a C-\ (not recommended)

See help: C-a ? (lists keybindings)

create new window: C-a c
change to last-visited active window: C-a C-a (commonly used to flip-flop between two windows)
change to window by number: C-a (only for windows 0 to 9)
change to window by number or name: C-a '
change to next window in list: C-a n or C-a
change to previous window in list: C-a p
see window list: C-a ” (allows you to select a window to change to)
show window bar C-a w (if you don't have window bar)
close current window: Close all applications in the current window (including shell)
kill current window: C-a k (not recommended)
rename current window: C-a A

split display: C-a S
jump to next display region: C-a tab
remove current region: C-a X
remove all regions but the current one: C-a Q