Mac OS X can do many jobs at once, dividing the processor's time between the tasks so quickly that it looks as if everything is running at the same time. This is called multitasking .
With a window system, you can have many applications running at the same time, with many windows open . But Mac OS X, like most Unix systems, also lets you run more than one program inside the same Terminal. This is called job control . It gives some of the benefits of window systems to users who don't have windows. But, even if you're using a window system, you may want to use job control to do several things inside the same Terminal window. For instance, you may prefer to do most of your work from one Terminal window, instead of covering your desktop with multiple windows.
Why else would you want job control? Suppose you're running a program that will take a long time to process. On a single-task operating system, you would enter the command and wait for the system prompt to return, telling you that you could enter a new command. In Mac OS X, however, you can enter new commands in the "foreground" while one or more programs are still running in the "background."
When you enter a command as a background process, the shell prompt reappears immediately so that you can enter a new command. The original program will still run in the background, but you can use the system to do other things during that time. Depending on your system and your shell, you may even be able to log off and let the background process run to completion.