An alarm clock is a time keeping device used to wake someone at a specific time usually from heir nights sleep. An alarm clock can also be used for other day time reminders. Alarm clocks tend to use sound to wake someone but others also use light, a radio alarm clock makes use of an integral radio to wake someone.
Alarm clocks can make excellent ironic or sarcastic gifts especially when bought for those that have trouble waking up in the morning, they're also good gifts for work colleagues who often come to work late. A travel alarm clock is basically a very small alarm clock that can often be folded in on itself so that it can be easily stowed away in a pocket or rucksack.