In general standard CCTV cameras are supplied in three voltages, 120VAC, 24VAC and 12VDC. Here is some advice to help you choose.
120VAC - These cameras are supplied with a 6 foot standard power cord like most appliances. The drawback is that you will need an outlet within 6 feet of where the camera is installed. When purchasing a camera that will be installed in an outdoor environmental housing the voltage of the camera should match the voltage of the housing.
24VAC - This is usually the voltage of choice in commercial installations and systems we have installed. The advantage is the camera is powered by an external plug-in low voltage power supply. The power supply is connected to the camera using low voltage wire 18 or 20 gauge (speaker wire). Siamese Cable is also available which is a cable with video coax and power cable under one jacket. Power does not need to be near the camera location. All power wires can be run to a central location near the monitor or the nearest outlet by the camera. 24VAC power wires can be run up to 350 feet. Large power supplies are available that will power many cameras. The drawback is the additional expense of purchasing the power supply.
12VDC - This selection should be made when the camera will be used in a vehicle or other locations where a large battery may be used to power the camera. 12VDC is supplied to the camera in the same manner as 24VAC but the distance of cable is limited due to voltage drop.