Introduction:
- Power over Ethernet(PoE) describes any of several standardized or ad-hoc systems which pass electric power along with data on twisted pair Ethernet cabling. This allows a single cable to provide both data connection and electric power to devices such as wireless access points and IP cameras.
- Power sourcing equipment (PSE) is a device such as a network Switch that provides (or sources) power on the Ethernet cable. The maximum allowed continuous output power per cable in IEEE 802.3af is 15.40 W. A later specification, IEEE 802.3at, offers 25.50 W.
- A powered device (PD) is a device powered by a PSE and thus consumes energy. Examples include wireless access points, IP Phones, and IP Cameras.
...
Here is a typical topology for PoE. The PDs are power devices connected to a PoE switch. 
FigureFigure. Power devices(PD) connect with PoE switch
Configurations:
...