Normally, you can exit
by exiting each shell
individually, or by using CTRL-a CTRL-
Instead of a
To reattach the session later on, start screen with the -r option. You'll be placed in exactly the same screen session that you were in before. All your processes are still running - for example, if you were in the middle of editing a file in one of your screen sessions, the editing session is still active. You can just continue where you were.
This is great because it means that not only can you keep active sessions all the time (even if you log out), you can also resume them from different terminals. So for example, I could go home, log in from there and pick up exactly where I left off.