A smart hub allows to manage all your wirelessly connected smart home gadgetry - smart locks, thermostats, room sensors, switches, lights, cameras, smoke alarms, leak detectors and more.
A smart uses a centralized app to control all these devices.
Good smart hubs are reasonable priced. They are compatible with more third-party devices, and supports all the major wireless protocols relied on by smart devices.
However, these are early days for the smart home / smart hub concept. Thus, these are early-stage devices.
The best smart-home hub: Samsung SmartThings Hub (multipurpose)
The best user-friendly smart hub: Wink Hub (widely compatible)