DVR's or Digital Video Recorders are video storage units for analog CCTV cameras. They are usually connected to the analog cameras with coax cabling and all cameras must have a cable directly connected to the unit.
NVR's or Network Video Recorders are video storage units for IP network cameras. While some have the option of having each camera wired directly to it via Cat 5e or Cat 6 network wiring, typically they are connected to the network infrastructure and they connect to any camera that's on the network whether wired or via or wireless WiFi cameras. Network camera systems or IP camera systems are the newer technology which provides for more versatility and higher quality than the older analog technology.