If no signal is specified, the TERM signal is sent. The TERM signal will kill processes which do not catch this signal.
Most modern shells have a builtin kill function.
Each pid can be one of four things. A pro- cess name in which case processes called that will be signaled. The pro- cess with pid n will be signaled. IFF a negative argument is given the signal must be specified first, otherwise it will be taken as the signal to send. Specify the signal to send.