[SunHELP] parent process & child process?

[Solaris Neophyte]

I don't follow what these are... 

this is what my text says...

"When a process is started, a duplicate of that process is created.  This new
process is called the child and the process that created it is called the
parent.  The child process then replaces the copy for the code the parent
process created with the code the child process is supposted to execute."

The last sentance confuses me the most.  Actually the entire thing confsues me.
 Why is a child process started anyways?  Why does the parent process spawn a
child process in the first place?  And what's all this code copy stuff all
about?

The next paragraph states...

"While the command is executing, the shell waits until the child process has
completed.  After is completes, the parent process terminates the child
process, and a prompt is displated, ready for a new command"

Why does this happen?  I guess it goes back to my confusion about why the child
process is created in the first place.
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