UPS is an abbreviation for Uninterruptible Power Supply or Uninterruptible Power Source. It's a battery used to power a PC or a hosting server to avoid the loss of information if the main power source fails for reasons unknown or becomes risky. A diesel generator pretty much self-explanatory. UPSs and generators are widely-used as a backup in data centers to back up the power supply and to ensure the consistent functioning of all of the machines located there. Due to the fact that the UPS works at all times, it provides the necessary power for the web servers to remain functioning until the generator starts and takes over. Using this type of a backup is a must for any data center or service provider that wants to keep their hardware and information intact in the case of a power surge or outage, because it provides them with plenty of time to react until the problem is resolved and the main power supply is restored.