Process is an abstraction of a running program. Each running program, called a process, has memory containing instructions, data, and a stack. The instructions implement the program’s computation. The data are the variables on which the computation acts. The stack organizes the program’s procedure calls.
A process has a group of associated threads that execute in the same, shared environment.
* A multi-threaded process is one that contains one or more threads.
* A single-threaded process is one that contains exactly one thread.
* A process may create a new process using the fork system call. Fork creates a new process, called the child process, with exactly the same memory contents as the calling process, called the parent process.