UPS is an abbreviation for Uninterruptible Power Supply or Uninterruptible Power Source. That's a battery used to power a computer or a server in order to avoid the loss of data in case the main source of energy fails for whatever reason or becomes unsafe. 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 web servers located there. Due to the fact that the UPS operates constantly, it offers the required power for the web servers to remain operational until the generator starts and takes over. Using this kind of a backup is vital for any data center or web hosting provider that wants to keep their hardware and info intact in the case of a power surge or outage, due to the fact that it provides them with ample time to react until the problem is resolved and the primary power supply is restored.