UPS is an abbreviation for Uninterruptible Power Supply or Uninterruptible Power Source. That's a battery used to power your personal computer or a web server in order to avoid the loss of data in case the main source of energy fails for reasons unknown or becomes unstable. A diesel generator is more or less self-explanatory. UPSs and generators are used as a backup in data centers to back up the power supply and to guarantee the continuous operation of all the web servers located there. Since the UPS runs at all times, it offers the required power for the web servers to remain operational until the generator starts and takes over. Using this sort of a backup is vital for any data center or hosting company that wants to keep their hardware and info intact in the event of a power surge or outage, due to the fact that it provides them with enough time to react until the issue is resolved and the primary power supply is restored.