screen is a very useful program which in effect gives you multiple console screens in the same window. It's handy if you're using a console rather than an xterm, or if you're connected to another machine, as it means you can do multiple things at once without having to move to another console or open another ssh connection. Even more useful if you're working remotely: If your connection falls over while you're still in a screen session, you can ssh back to the machine and reattach the screen (with screen -r) and get straight back to where you were, rather than having to start over.>
You can also manually detach the screen, with screen -d (detach screen), or by hitting Ctrl-A then D while in a screen session. This leaves the screen running in the background, and it can be useful for running background jobs. Or you can leave a screen session connected to another machine: Run screen before you open the ssh session, then instead of logging out when you're done, detach the screen, and then just reattach it (screen -r) when you need to connect to that machine again. The downside of this is that it does have security implications...!