An intervalometer is a device which counts intervals of time. Such devices are commonly used to signal, in accurate time intervals, the operation of some other device.
In photography, intervalometers are used to trigger exposures. This is often done for a time-lapse series. It may also be used to take, or begin taking, picture(s) after a set delay.
Other names include interval meter, interval timer and timer remote.
Read more on intervalometers at Wikipedia
Buy intervalometer timer remotes at DealExtreme (referral link)