Speed and features. A USB-C hub connects to a single 4K (or 1080p) display, and provides a mix of ports: USB-A, SD card slots, and so on. You can usually plug your laptop’s USB-C power cable (if it uses one) right into it.
A Thunderbolt dock supplies even more bandwidth for more ports. There are two key differences: It has enough bandwidth to drive a pair of 4K displays, and many docks come with their own power supply that can charge your laptop as well as your phone. All that occurs via the Thunderbolt cable that connects your laptop to the dock.