Power over Ethernet (PoE) allows devices to use an ethernet connection as a power source, most commonly found in IP phones. The ethernet cable provides the power for the device to operate as well as network connectivity.
Wikipedia
Power over Ethernet, or PoE, describes any of several standards or ad hoc systems that pass electric power along with data on twisted-pair Ethernet cabling. This allows a single cable to provide both a data connection and enough electricity to power networked devices such as wireless access points (WAPs), IP cameras and VoIP phones.