Power over Ethernet (PoE)
Technology that allows VoIP over IP telephones to be powered using regular network cables rather than power adapters which plug into the wall. This has the advantage that you can power all the phones in your office from a single source and makes installing a battery backup unit much easier.
Uninterruptible Power Supply (UPS)
Is a device that powers your equipment when you lose power at the office. The system has a built in battery which keeps your network devices operational when the power goes out.
The easiest way to protect your phone system from a power outage is to power all the phones using a Power-over-Ethernet switch that would normally be connected in the back room where your router and cable/DSL modem is located. This has the advantage that all your phones are drawing power from a single source which you can backup using an uninterruptible power supply (UPS). All you need to do is plug in your PoE switch, router, and DSL/cable modem into a sufficiently powerful UPS device so that when the power goes out all your phones remain up and running.