fork() creates a new process where exec() not....
exec() shares memory of parent here fork() process operates 
independently...

fork: Create a new process

exec: replace a process's objectcode with contents of file
(executable)

fork() creates a child process with most of the attributes
of the parent process. Both parent and child processes are
executed simultaneously.
exec() replaces the current process with a the executable
pointed by the function. Control never returns to the
original program unless there is an exec() error.

processes began through a process called "fork and exec".
that is when one command starts another, the child process
first forks , the kernel copying over pages of memory from
the present process to a new location for the child process.
The child then execs, executing the new command and
overwriting the data