In computing, when a process forks, it creates a copy of itself, which is called a "child process." The original process is then called the "parent process". More generally, a fork in a multithreading environment means that a thread of execution is duplicated, creating a child thread from the parent thread.
Under Unix and Unix-like operating systems, the parent and the child operations are selected by examining the return value of the fork() system call. In the child process, the return value of fork() is 0, whereas the return value in the parent process is the PID of the newly-created child process.
The fork operation creates a separate address space for the child. The child process has an exact copy of all the memory segments of the parent process, though if copy-on-write semantics are implemented actual physical memory may not be assigned (i.e., both processes may share the same physical memory segments for a while). Both the parent and child processes possess the same code segments, but execute independently of each other.
Reference:
http://en.wikipedia.org/wiki/Fork_%28operating_system%29
