One of the attributes of a process is its controlling terminal. Child
processes created with fork inherit the controlling terminal from
their parent process. In this way, all the processes in a session
inherit the controlling terminal from the session leader. A session
leader that has control of a terminal is called the controlling
process of that terminal.
You generally do not need to worry about the exact mechanism used to allocate a controlling terminal to a session, since it is done for you by the system when you log in.
An individual process disconnects from its controlling terminal when it
calls setsid to become the leader of a new session.
See Process Group Functions.