user command,system calls and func like...
 

What is the difference between user command,system calls and func like pthread_create under pthread.h lib of c in red hat?

also plz explain how manual is divided in red hat???????

To see the manual divisions, look at the manual page for the "man" command. i.e. do this in a terminal:
The rest sounds very much like a homework question. You should examine your course notes.