An NVR works with IP cameras: it receives video over the network, stores it on hard drives and lets you view, search and export recordings.
When choosing an NVR, check how many cameras it supports (channels), how many drives it holds and the maximum resolution it handles. Some NVRs have built-in PoE ports so cameras connect to them directly.
Example
A 16-channel NVR with two hard drives for 12 office cameras, leaving 4 channels spare for future expansion.