Network video recorder(NVR)
A network video recorder, or NVR, is a device on site that receives the streams from IP cameras over the network and records them to its own hard disks. Footage is viewed and exported from the recorder itself, so it is both the storage and the single point of failure for every camera attached to it.
An NVR differs from the older DVR in what it accepts: a DVR takes analog camera signals and digitizes them, while an NVR takes streams that the IP cameras have already encoded. Both keep the footage on disks inside the box, and both are usually sized once at installation.
The recorder sets the limits of the system. Retention is bounded by the disks fitted, the number of cameras by the channels licensed, and remote viewing by whatever port forwarding or vendor app the recorder supports. When the recorder fails, is stolen or is unplugged, the footage on it goes with it.
A cloud VMS replaces the recorder with a service: cameras stream to the cloud, retention is set per camera rather than per box, and every site is viewed on one login. A site with too little upstream bandwidth, or a policy that footage never leaves the building, is the case where the recorder still wins.
Many sites run both during a transition. A recorder that publishes its cameras as RTSP streams can be the source the cloud records from, so the archive on the recorder stays where it is while new footage is kept off site.