One of the many jobs assigned to system administrators is making sure the programs of users are running properly. This task is made more complex by the presence of other programs running concurrently on the system. For various reasons, these programs might fail, hang up, or otherwise misbehave. Understanding how the UNIX® environment creates, manages, and destroys these jobs is a crucial step in building a more reliable system.
This is a great read about the life cycle of a process running under the UNIX operating system. Read more @ IBM developerWorks