Power over Ethernet (PoE) is a standard that allows electrical power to be transmitted over your network using the same cables that transmit your network data. In order to take advantage of POE, first you must have a POE network switch. A POE switch can deliver power to attached devices. Many network attached devices, such as computers will not use the power, but some POE enabled peripheral devices such as wireless access points and VOIP phones can get all of their power via the POE enabled network. This allows you to discard those unsightly wall warts normally used to power peripheral network devices. PoE also avoids the problem of having a network port available, but no AC receptacle nearby.
What is the difference between PoE and PoE+?
PoE, which is covered by the 802.3af standard can supply up to 15.4 watts of power to a network attached device, whereas PoE+ (i.e. 802.3at) can supply 25.5 watts to a device. Whether you need the higher power of PoE depends on the power consumption of the devices you plan to use.