Question

What is job? distinguish between foreground and background job?

Answer 1

In a foreground job, all of your input is directed to the process's stdin (including the SIGINT that is generated when you press CTRL-C, and that's why you can still kill the process if it's in the foreground).  In a background job your input is directed to the shell process instead, and that is why you see the shell prompt and you can execute commands.