With PoE, a network cable (such as Cat6) carries electricity along with the data. An IP camera, a Wi-Fi access point or an IP phone then needs no separate power outlet or adapter.
Power usually comes from a PoE switch or a PoE injector. Installation gets simpler, fewer cables are needed, and if the switch is on a UPS, the cameras keep working during a power cut.
Example
Eight IP cameras in a yard are powered from a single PoE switch — no power outlets had to be installed on the poles.