Screen is an application that is part of the default distribution of many types of Linux.
It allows you to create virtual terminal screens which can be detached and run in the background, then re-attached and brought to the foreground at will. In MS-Windows terms, think of it like minimizing a window. It's still there and running, you just aren't looking at it on your desktop. Screen does the same thing for terminals like Xterm, telnet or ssh. You should stop reading this now while you figure out where to get and install it from.