Power Over Ethernet

I'm a bit confused about Power Over Ethernet :confused:

I'm looking to buy a couple of cameras to start with that have POE capability, it says on the website - Power: DC 12V 1A/POE.

I was looking at getting a 8 PORT PASSIVE POE POWER INJECTOR so that I've got some future expansion ability, it says on the website - 48 volt, 60watt power supply included (100-240v autosense, 50/60hz, UL / CE approved)

So I beleive that the injector will supply 48volts over ethernet but do POE devices automatically drop the 48v down to the required voltage, in this case 12v?

Thanks for your help

Simon