A handy reference, A guide to GNU Screen, appears in Red Hat Magazine:
The same way tabbed browsing revolutionized the web experience, GNU Screen can do the same for your experience in the command line. GNU Screen allows you to manage several interactive shell instances within the same “window.”
The killer feature of screen, in my mind, is the ability to launch a long-running task, disconnect and reconnect to it later. For a consultant on the go, or when your wireless isn’t reliable (or the dog pulls the ethernet out of the wall), screen lets you reconnect and pick up where you left off. It’s handy for setting up a set of screens on a client site and then using only one secure tunnel to connect to them and examine the state of various applications. This is a tool everyone should learn and keep in their toolkit.