One of the best tools out there if you do sysadmin stuff.
It is so worth reading the man page there are many many features to this wonderful tool. One great one that I never really fully mastered is the copy and paste mode. What follows are the copy and paste basics.
At least enough to get started:
C-a Esc
Enter copy mode, you can now use vi like bindings to navigate the buffer. Hit space to set the first mark, and then navigate (it will highlight) and hit space to set the second mark and throw the contents between the marks in the copy buffer.
C-a >
Write the copy buffer out to a file /tmp/screen-exchange
C-a ]
Paste into the current screen's stdin buffer.
This little bit alone makes for a greatly enhanced screen experience. But read through the man page there are tons of little gems. This is evidenced by one of the entries the Bugs section of the man page:
"- A weird imagination is most useful to gain full advantage of all the features."