A program in execution is called process. Each process will have unique process id and this process status of any process can be viewed by using ps ( process status) command. if a process starts its job,  there are more reasons where the process can do more than what it was planned to do. Following are some of the scenarios where we can occur
a) Looping continously
b) generating memory Leak
c) Creating a dead lock scenario

All programs can be communicated through singals and the same can be handled through singnal handlers. Any program cannot handle the singal of ( kill -9) which is an abnormal shutdown. We have to think several times before we use this command. preferably we can user the kill command to allow the process to do its normal routines before it shutdowns. In case if kill command is not worth full then better to user the 'kill -9 PID' Command to kill the process.