Transients and spikes are extremely fast, high-energy-density power surges that occur for periods of a few thousandths of a second and sit on the back of the normal mains sinewave power.
They vary in voltage size from zero to up to over several thousand volts or even more and are a result of a sudden stored energy release. They are very common during thunder and lightning storms but can also be as a result of load shedding as well as from inductive loads such as motors. Even a fridge or freezer can cause them.
Spikes and transients can easily damage sensitive loads and corrupt data. They will even damage household loads which is why houses located near industrial areas (which generally generate a lot of spikes and transients back into the mains supply) often experience more frequent appliance failures.
An online double-conversion uninterruptible power supply or UPS will prevent any transients or spikes on the live mains supply from affecting the load by blocking them on the rectifier input.
