A computer has a number of input devices such as keyboard and mouse. These devices
contain a trigger which can be used to send a signal to the OS.
For the request input mode, the input is provided to the program only when the user triggers
the device (e.g. entering a number then pressing <enter>). The program that requests this
input comes to a halt until the input is entered.
Since most computer systems have more than one input device, each of these devices can
be triggered at an arbitrary time by the user. Each trigger from these input devices generates
an ”event” that is put in an ”event queue” which can be examined by the user program. For
the event input mode, the program may be executing many other operations (i.e., it does not
come to a halt) while waiting for an event to be triggered.