Glenn Fleishman, writing for TidBITS, does an excellent job walking through the various standards that ultimately connect to the new MacBook Pro via USB-C. Bookmark the link, pass it along.

One point worth highlighting:

This may all seem confusing initially, but it should pass quickly because everything on the market for USB and DisplayPort over USB-C today should work with Thunderbolt 3. The main group that will be disappointed are those who buy Thunderbolt 3 peripherals and expect them to work with a 12-inch MacBook, which doesn’t extend USB-C support to Thunderbolt. We can hope that Apple makes Thunderbolt 3 standard across the entire Mac line.

In addition, there appears to be a compatibility issue with support for older Thunderbolt 3 peripherals. Read about it in this post.