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Power over Ethernet or PoE technology describes a way to power a camera using the very same Ethernet cable used to connect the camera to the network. The big advantage is that you do not need a seperate power supply and 110VAC to power a camera.
The standard for PoE is 802.3af and provides up to 15.4 W of DC power (minimum 44 V DC and 350 m to each camera.
Your home router does not have this feature built in. You either have to get a switch that has the PoE feature or what is called a PoE injector.
A PoE injector plugs into your AC near your router, a cable goes from the injector to your router and the Ethernet cable from your camera plugs in the other connector. This powers only one camera per injector. Prices vary from $10-100 depending on brand name, but they all pretty much do the same thing.
A PoE switch makes more sense if you are running low on open ports on your router and you plan on having more than one camera. A typical home/small business PoE switch looks like any other switch, but will have designated ports for connecting PoE devices, usually half will be PoE, half will be standard ports. Switches just plug into an open port on your router and are sort of like an intelligent extension cord. If you are looking for an inexpensive PoE switch, a Trendnet TPE-S44 can be found for about $50, has 8 ports, 4 of which are PoE.
Inexpensive consumer cameras in many cases are WiFi and this does not apply. The negatives on WiFi are that you have to have an A/C outlet to plug the camera into. Indoors this may not be a problem, but outdoors, finding a convenient outlet is difficult, and running low voltage ethernet may be easier than setting up a 110VAC GFI circuit outdoors, not to mention having a transformer plugged into it to power the camera. Also, while cameras may connect to WiFi, it may not get a strong enough signal to achieve the frame rates advertised, especially when mounted outdoors.