Why should you care about process IDs or job numbers? Actually, you could probably get along fine through your UNIX life without ever referring to process IDs (unless you use a windowing workstation—as we’ll see soon). Job numbers are more important, however: you can use them with the shell commands for job control. 
You already know the most obvious way of controlling a job: create one in the background with &. Once a job is running in the background, you can let it run to completion, bring it into the foreground, or send it a message called a signal.
The built-in command fg brings a background job into the foreground. Normally this means that the job will have control of your terminal or window and therefore will be able to accept your input. In other words, the job will begin to act as if you typed its command without the &.
If you have only one background job running, you can use fg without arguments, and the shell will bring that job into the foreground. But if you have several jobs running in the background, the shell will pick the one that you put into the background most recently. If you want some other job put into the foreground, you need to use the job’s command name, preceded by a percent sign (%), or you can use its job number, also preceded by %, or its process ID without a percent sign. If you don’t remember which jobs are running, you can use the command jobs to list them.
A few examples should make this clearer. Let’s ...