UPS is an abbreviation for Uninterruptible Power Supply or Uninterruptible Power Source. It's a battery used to power a PC or a server in order to avoid the loss of information when the main power source fails for whatever reason or becomes unstable. A diesel generator is more or less self-explanatory. UPSs and generators are widely-used as a backup in data centers to back up the power and to guarantee the continuous operation of all the machines located there. As the UPS works at all times, it offers the desired power for the web servers to remain working until the generator starts and takes over. Using this type of a backup is crucial for any data center or web hosting provider that wants to keep their hardware and information intact in case of a power surge or outage, since it offers them lots of time to react until the issue is resolved and the main power supply is restored.