Up till now we have pretended that the UNIX system we were using was only used by one user and only one program was running at a time. In fact the computer you are using can be concurrently used by many users and be appear to be running many programs at the same time.
There are a number of programs running on most UNIX systems designed to ensure smooth operation. These programs will run even if no one is using the system and most of these programs are not of real interest to us as they only help the operating system manage cache, virtual memory and logging of system messages. But some like cron, lpd and xdm have a direct relevance to us. We will later see how.
When we log into the system our shell is started. Every user that logs in has his shell started at login, so there is one shell running for each user simultaneously.