As you can see by (Diagram 1) The camera and transmitter both need power. The camera sees an image, sends it to the transmitter, and the transmitter sends the signal out to the air. The receiver picks up the signal and outputs it to a TV / Computer / Digital Video recorder/ This is a basic diagram many wireless cameras and transmitters are very small and the power is provided to both from one source. A good example of this is an Hidden wireless camera. IE: A clock radio wireless camera is powered by plugging in the clock. The camera and wireless transmitter are provided power by the clock radio internally.
A wireless receiver has only one function. After the camera and wireless transmitters have provided the wireless video signal the receiver collects this signal and routes it the Monitor, TV, VCR , DVR or PC (or alternative recording or viewing device). See diagram 2 .
As you can see in Diagram 2 The receiver accepts the wireless transmitters signal and then out puts it to your TV, VCR, Monitor or PC. The receiver needs only power and a Device to view and or record the Signal /Video.
Video Transmitters / Receivers
Video Transmitters can be obtained separate from cameras. If you have a wired camera now you can turn it into wireless by adding a transmitter and receiver. Instead of the wire from the camera to the recording device or monitor the wireless signal will send the video. Again you will need to provide power to the camera and the transmitter. Most transmitters and receivers are sold as a package but some are not. Be sure to check with the supplier.