A controlling terminal has an associated ''foreground ProcessGroup''; only processes in this group are allowed to ''read'' from the terminal (though all processes which may have that terminal currently open are allowed to ''write'' to it). Processes in other process groups sharing the same controlling terminal are said to be in the ''background''; if any processes in those groups try to read from this terminal, they will get a SIGSTOP signal which causes them to be suspended until their process group is brought into the foreground. 

  

 Also, when you hit CTRL/C, the SIGINT signal is only sent to processes in the foreground process group.