Power over ethernet lets you add a DC voltage source to the unused pairs in your ethernet cable. This power can be used to power devices that are poe compatible by just plugging the cable into them. Other devices can be powered by using a “tap” to break the DC pairs back out of the cable. POE is a good choice for powering devices in remote locations. A router can be placed on a roof right next to its high-gain antenna, reducing signal loss, without having to run a separate AC line. Plugging the DC “injector” into a UPS will keep dedicated VOIP phones functioning during a power outage. Terry Schmidt has written a nice guide covering the theory behind scratch building poe injectors and taps. It also has photos and descriptions of other peoples projects along with tips on how to keep from turning your router into a pile of plastic goo.