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The system call setsid is used to create a new session containing a single (new) process group, with the current process as both the session leader and the process group leader of that single process group. Process groups are identified by a positive integer, the process group ID, which is the process identifier of the process that is (or was) the process group leader. Process groups need not necessarily have leaders, although they always begin with one. Sessions are identified by the process group ID of the session leader. POSIX prohibits the change of the process group ID of a session leader.
The system call setpgid is used to set the process group ID of a process, thereby either joining the process to an existing process group, or creating a new process group within the session of the process with the process becoming the process group leader of the newly created group. POSIX prohibits the re-use of a process ID where a process group with that identifier still exists (i.e. where the leader of a process group has exited, but other processes in the group still exist). It thereby guarantees that processes may not accidentally become process group leaders.
The system call kill is capable of directing signals either to individual processes or to process groups.
Read more about this topic: Process Group