In 1957, Ken Olsen and a group of his buddies got tired of the sales practices of IBM corporation and started their own computer company, Digital Equipment Corporation. They started building much cheaper mainframes which became really popular. Unix was originally designed on a Digital Equipment machine.

[ rfc791.ORG : Unix Help : Unix Processes ]

ps, kill - process management programs
Unix is a multitasking operating system, which means it can run multiple processes at once. A process is a running program.

The first process you'll see when you log in is the shell. The shell allows you to start other processes. Most shells allow you to start a process in the background using the '&' operator.
[18]Tethys% rm -fr * &
[1] 90463
This deletes all the files in the current directory in the background. Being in the background as it is, you can interact with the shell as the background job does it's thing. The [1] tells you the job number so you can pull the job back into the foreground if neccesary. The 90463 tells you the Process ID, which is the system-wide unique number that identifies your process. You can see all the processes, complete with their IDs by running the ps command.
[19]Tethys% ps
90280  p1  Ss     0:00.08 -csh (csh)
90463  p1  T      0:00.20 rm -fr *
90470  p1  R+     0:00.00 ps
Here we see our shell (csh), our background file deletion (rm), and the process listing our processes which we just ran (ps).

Processes under Unix can intercommunicate by sending signals - special messages that command a process to perform a certain action.
There are several signals that can be sent to a process. A program can decide how to react when it receives a certain signal, however usually a given signal will produce the same sort of result in any program.
You can send signals to processes yourself using the kill program.

[20]Tethys% ps
90280  p1  Ss     0:00.08 -csh (csh)
90463  p1  T      0:00.20 rm -fr *
90470  p1  R+     0:00.00 ps
[21]Tethys% kill -KILL 90463
The -KILL part of the previous command tells the kill program that the signal you want to send it is the KILL signal. Here are some of the signals that processes usually can receive: