Glenn Fleishman, TidBits:
The reason confusion afflicts this space is that a USB-C port on another computer may support just USB, USB plus display and networking protocols, or all of that plus Thunderbolt 3. The 12-inch MacBook’s USB-C port, for instance, natively supports USB 2 and USB 3 along with DisplayPort and, via adapters, VGA, HDMI, and Ethernet connections, but not Thunderbolt 2 or FireWire.
The summary for potential late 2016 MacBook Pro owners is that all current USB-C devices, cables, and adapters should work when plugged into a MacBook Pro’s Thunderbolt 3 ports. However, Thunderbolt 3-specific devices won’t work with computers and other devices like the 12-inch MacBook whose USB-C ports are less capable. Now, let’s drill down into details.
The adapters available for Thunderbolt 3 are also a little confusing. I suspect articles like these are necessary because both the USB and Thunderbolt ports have changed, all in one and at the same time. Also, USB-C is so multipurpose and adaptable that its myriad capabilities can be hard to understand; it isn’t like going from USB 2 to USB 3, where the only incompatibility you’d notice was a reduction in speed.