To examine all processes, type ps -e. A user can have many processes running at one time. Normally, a process will run in the foreground, by default. This means that the processor will devote its energy to completing the user's request. Until the request is completed, the processor will not accept input from the user.
Most tasks are important enough or quick enough to be run in the foreground with no problems.
Some tasks, though, are long or tedious. With such tasks, it is possible to have the processor work on these at a lower priority than jobs that are in the foreground. It is possible, in other words, to put a process in the background. The processor will still work on background tasks, but it will not devote so much time or effort to getting these tasks completed. The advantages of doing this are that the tasks in the background still get completed, and the processor is freed up to perform more immediate tasks.