PoE begins with a CAT5 “injector" that inserts DC Voltage onto the CAT5 cable. The injector is typically installed in the wiring closet near the Ethernet switch or hub. In some cases, the Ethernet switching equipment itself is capable of playing the role of power injector on some or all of its ports.
Some wireless APs accept the injected DC power directly from the CAT5 cable through the RJ45 jack. These devices are considered to be "PoE-compatible" or "Active Ethernet Compatible."
Devices that are not PoE-compatible can be converted to PoE by way of a DC “picker” or “tap” that is sometimes called an Ethernet “splitter”. This device picks-off the DC voltage that has been injected into the CAT5 cable by the injector and makes it available to the equipment through the regular DC power jack.
It is important to note here that Cisco Systems until recently did not support 802.3af standard PoE in its product line. Because of this, products that were 802.3af compliant could not be powered by Cisco equipment, such as Catalyst switches, that had PoE capabilities.