These pages are deprecated (latest changes date from end 2001). More recent information may be found at http://www.coresequence.com/training.php. I leave the old pages here as they are because lots of people are still visiting them (eventhough they aren't spell-checked), and because the basics are still useful.
On a single-task system such as MS-DOS, you enter a command and wait until the system returns the prompt, indicating that it's your turn again to do something. If the command takes a long time to execute, this can be rather annoying.
In Unix, however, there are ways to enter new commands in the "foreground" while one or more commands (processes) are running in the "background", enabling the system to process more than one job per session. Depending on your system and shell, you may even log off and come back when the job is done.