Screen is a very powerful program that allows multiplexing several interactive shells. The sessions can be attached and detached, this is especially useful for sessions that are used on a remote sever: One can log on from a different computer, attach the screen session and continue working in the same environment which was last encountered.
Most important commands 
Starting a screen session:
screen -S sessionname
Attaching an already running screen session:
screen -r sessionname
list all running screen sessions
From inside a screen session, all commands are started with Ctrl-a and then another key. Detach session: Ctrl-a + d Switch to next window: Ctrl-a n Switch to previous window: Ctrl-a p