Ever hear of leap years? Of course you have. Especially if you read this blog. Ever hear of leap seconds?
The length of a second of time has an official definition. Yes, it's one day divided by 24 hours divided by 60 minutes divided by 60 seconds, but the official definition of an atomic second is 9,192,631,770 oscillations of a Cesium atom at zero magnetic field. This gives a very precise length of time, and is used by the U.S. Naval Observatory as the standard for measuring time.
But there's a problem with that. Seems the earth is slowing down a bit. This is caused by the braking action of the earth's tides, similar in a way to the effect that water sloshing around in a tank can affect its movement. So an actual second (i.e. that 1 day / 24 / 60 / 60 ) isn't quite the same length as an atomic second, and it's off by about 2 milliseconds per day. This means that every 500 days or so, the atomic clocks are off by one second. So they add or subtract a second from the atomic clocks as appropriate to compensate for this (typically either at the end of December or June).
More details on this can be found at the USNO site.