"Battery Backup' means different things in different contexts but it generally means that critical data like disk buffer caches are preserved through power failures and that when power is restored, the cache can be flushed to disk without loss of data. This is a fairly common feature of disk arrays and a few computers had feature like that to hold memory and registers stable until normal power could be restored. An Uninteruptable Power Supply is just what its name implies. It serves as an alternate source of typically AC power until normal power can be restored. The devices powered by the UPS should never even know anything has happened. The ideal situation is a standby generator to supply power for extended outages coupled with a large UPS or many smallers UPS to supply power in the brief period while the generator is coming on-line.
UPSs use batteries; the more batteries/battery capacity you have, the longer the backup time (and the more space you eat up with UPSs. You can have a battery setup that doesn't use a UPS.
There's three basic UPS types 1) Standby - sense the power. On blackout, supply power from a battery. 2) Hybrid or line interactive - sense the power. On black out, supply power from a battery, on brown-out make up power from the battery, sometimes. 3) Continuous duty - take power from the wall; charge a battery; invert & rectify; supply "clean" power to equipment all the time.
In general, UPSs are designed power equipment for small amounts of time. They expect to be backed by something like a diesel or gas generator.
UPSs don't like power hungry equipment with large current draws (printers, monitors, hair dryers, room fans, heaters, air conditioners).
In manufacturing and telco facilities that require lots of uptime, you'll see big battery implementations. In fact, a lot of the equipment is set up to run DC directly, instead of AC. The equipment is always fed by the batteries; the wall power charges the batteries.
Continuous Duty / battery all the time provides clean, regulated power. That's why folks who use expensive equipment migrate towards continuous duty/battery all the time.
Current starvation (not fixed with standby UPSs and not fixed well with line interactive UPSs) doesn't mix well with circuit boards, though in modern manufacturing, some vendors have built capacitance, battery schemas, and power supplies that seem to have "better inertia" getting through dirty power.
For "smaller" installations, use UPSs. For bigger installations, hire/contract an EE who understands power/power distribution and uptime.
