Your DVR Decorder or player. Which is like a CPU for your home security system. It connect all the camera to the system, run a program, so you can see the images/ videos on your house when you are not around. It has the recorder system and also hard disk for the data storage. Just make sure your setting is right to record the most recent 2 months or 3 months video.
PDF below show more details of the function of a DVR. There are many type/ model/ brand DVR in the market. Hybrid DVR, or NVR Surveillance is one of them.