Closed Circuit Television (CCTV) has been around since the 40's. The principle behind CCTV works the same as your TV works at home with a minor difference. The channels you watch at home are publicly broadcast where as CCTV is strictly limited to the monitors or TVs connected to the network.
An unlimited number of cameras stream video and in some cases audio to a patch where the streams are sorted and sent to the appropriate monitors. This technology has many applications in recent times it has been used most predominantly for crime prevention or evidence applications.
CCTV is also used to monitor traffic and can be fitted with motion sensors which can detect if a car has stopped due to a collision or break down or if it is going in the wrong direction. Using a computer this can then alert who ever is monitoring the camera and action can be taken.
The long term storage and archiving of CCTV feeds can be an issue as video can amount to very large file size. File size is dependant on the type of compression used, the size of frames, frame rate and much more. Tapes used to be used to record CCTV however they have recently been replaced with digital Hard Disk Drive (HDD) recording as this takes up a lot less physical room is easier to archive and has the ability to retain the quality of the original image indefinitely.