We all do. According to a study by IBM, the average computer experiences around 120 power-related abnormalities a month. Most of these, of course, aren't complete outages, but they do nevertheless pose a problem for the well-being of your little electronic friends. A good percentage of these abnormalities are flat-out outages, where your computer just runs out of juice. Over-voltage is any voltage greater than what normally should be present on any given power line.
Surges are the true enemy when talking about over-voltage conditions.
Spikes can cause damage over long periods of time, but surges are mostly to blame for hardware destruction. Sags, brownouts, and blackouts are all under-voltage conditions. But we all know that there is more to fear than lightning. In fact, if you live in a bustling city center, the biggest fear for you is probably under-powered utilities. That's certainly the case where I used to live.