Alarm clock has gone a long way already. It had also improved for the better.
From the name alarm clock it you can get what is it for. This are designed to alarm a person by making sounds or noise in a specific designated time. Like this is made for those people who need to be reminded for a certain appointment or schedule and for those who can intend to over slept or need a nap and need to wake up in a specific time. All it need for an alarm clock to work is to set the time of your desired that it should to it purpose to alarm and make some noise or sound for one to be reminded.
The most common type of this kind of alarm clock that we see is those that have one or more of two bells that will ring because of mainspring that will trigger the gears to move back and forth from the bells.
But now the alarm clock got better and much more high tech. There are now called online alarm clock where in you can set it to your desire way to sound ad look like. Where in one can make use of their favorite songs to wake them up. This might a good thing to wake up in a good mood because of your favorite song is the one waking you up. And you can also used scary sounds that would spoke you that can wake you up. But still you have the choice how it is done.