Difference between DVR / NVR

Network Video Recorder Digital Video Recorder
NVRs utilize 'off the shelf' PCs with a standard Ethernet interface for receiving video and controlling PTZ devices DVRs utilize proprietary PC based devices containing analog to digital capture cards.
NVR technology allows secure, direct access to the video feeds independent of the NVR itself. Remote video access is only available through the DVR.
NVR failures can be repaired or replaced by the user, PC manufacturer or retail point of purchase very quickly. DVR failures require the DVR manufacturer to service the unit or replace it
NVRs can be replaced and upgraded in a matter of minutes. Performance can be significantly augmented over short periods of time. This allows new technology offerings to be immediately implemented. DVR performance is basically fixed. It can not be easily augmented and has physical limits. Performance augmentation requires premium cost components sold by the manufacturer.
Retired NVRs are completely viable PCs which can be sold or used for other services. DVRs require manufacturer approved components and accessories.
NVR maximum resolution is effectively unlimited. As camera resolutions continue to increase, NVRs can receive and decode the images without modification. DVR maximum resolution is limited by analog camera technology and capture card design. Native analog resolutions have approached their limits.