OS X has always been quite versatile in the network interfaces it can use. As well as the conventional interfaces like Ethernet and Wi-Fi, OS X could also network over ports such as USB and FireWire. Now with Mavericks, Apple have enabled networking over Thunderbolt.
If you connect two Thunderbolt enabled Macs together using a Thunderbolt cable you will find a new network interface called “Thunderbolt Bridge” pop up in the network preferences. Here you can configure your normal TCP/IP settings.
Once setup, you then have a normal network connection between the two machines.
Running Network Utility shows a 10GB link speed. On the new machines you get 20GB.
As a quick test, I copied a 5GB file between two laptops, which took 40 seconds. This could prove really useful if you need a quick way to transfer some large files between two machines.