Yet another good article from Martin Streicher , Editor-in-Chief, Linux Magazine that explains UNIX Multitasks (many concepts applies to Linux as well).
From the article:
On UNIX systems, each system and end-user task is contained within a process. The system creates new processes all the time and processes die when a task finishes or something unexpected happens. Here, learn how to control processes and use a number of commands to peer into your system.
UNIX has many moving parts. It has system services, devices, memory managers, and more. Luckily, most of these complex machinations are hidden from view or are made convenient to use through user interfaces, such as the shell and windowing tools. Better yet, if you want to dive in, specialized tools, such as top, ps, and kill, all are readily available.