If you look at the processes on your Linux system, you will see that there are rather a lot. For example, typing ps shows the following processes on my system:
$ ps PID TTY STAT TIME COMMAND 158 pRe 1 0:00 -bash 174 pRe 1 0:00 sh /usr/X11R6/bin/startx 175 pRe 1 0:00 xinit /usr/X11R6/lib/X11/xinit/xinitrc -- 178 pRe 1 N 0:00 bowman 182 pRe 1 N 0:01 rxvt -geometry 120x35 -fg white -bg black 184 pRe 1 < 0:00 xclock -bg grey -geometry -1500-1500 -padding 0 185 pRe 1 < 0:00 xload -bg grey -geometry -0-0 -label xload 187 pp6 1 9:26 /bin/bash 202 pRe 1 N 0:00 rxvt -geometry 120x35 -fg white -bg black 203 ppc 2 0:00 /bin/bash 1796 pRe 1 N 0:00 rxvt -geometry 120x35 -fg white -bg black 1797 v06 1 0:00 /bin/bash 3056 pp6 3 < 0:02 emacs intro/introduction.tex 3270 pp6 3 0:00 ps $If my system had many CPUs then each process could (theoretically at least) run on a different CPU. Unfortunately, there is only one so again the operating system resorts to trickery by running each process in turn for a short period. This period of time is known as a time-slice. This trick is known as multi-processing or scheduling and it fools each process into thinking that it is the only process. Processes are protected from one another so that if one process crashes then it will not affect any others. The operating system achieves this by giving each process a seperate address space which only they have access to.