Apple's Thunderbolt 3 protocol uses the USB-C connector type. Thunderbolt 3 is also compatible with USB-C, so a Thunderbolt 3 port will work with both a Thunderbolt 3 and USB-C connector. Similarly the Thunderbolt versions 1 and 2 use a standard Display Port connector.
USB-C, technically known as USB Type-C, is a 24-pin USB connector system, which is distinguished by its rotationally-symmetrical connector.
The USB Type-C Specification 1.0 was published by the USB Implementers Forum (USB-IF) and was finalized in August 2014. It was developed at roughly the same time as the USB 3.1 specification.
A device that implements USB-C does not necessarily implement USB 3.1, USB Power Delivery, or Alternate …