First talk about the kill command to kill it by pid (Process ID) process, a process to get pid, we can use the ps (process status) command, by default, the kill command sends a signal to terminate the process is 15, but some reason this process does not signal, in which case, the signal can be used to force kill 9, signal 9 is mandatory to perform signal can not be ignored.
The killall and pkill are using the name of the command to terminate the process of effective means, different from the way they kill to kill the process by pid, it can be understood as droves of killing process, of course, the risk of doing so is relatively improved a lot. Of course, they may also send a corresponding signal to terminate the process.
For example, we want to kill related processes mysql, we can use pkill -9 mysqld mysql to kill all related processes, more convenient, but it may inadvertently remove programs.