AirPort Time Capsule 802.11ac
It's backup, redesigned from the ground up.
Background
The June 2013 release of the 5th generation models features a name change to Airport Time Capsule, and a redesign with measurements 3.85 inches (9.8 cm) square, and 6.6 inches (17 cm) high. The square dimensions echo the size of both the latest AirPort Express and Apple TVs (2nd generation onwards), just with the height being significantly higher. The 2013 models feature the same I/O ports on the back as previous generations, and come in the same capacities as the 4th generation of 2 TB and 3 TB, but have introduced the newest Wi-Fi standard 802.11ac. The new AirPort Extreme released at the same time is exactly the same in dimensions and I/O ports, just without the internal hard drive of the AirPort Time Capsule.