Hue is a historical place located towards the Center of Vietnam. It was the Center of the country during the rule of the Minh Dynasty. It is also not very far away from the DMZ, which was the line dividing North and South Vietnam during the war.
I took the night bus from Phong Nha and stayed at two different places in Hue. First was a beautiful resort called the beach bar. This was located half an hour away from Hue on Thuan An beach. This is a perfect getaway from the hustle and bustle of Hue. Be sure to find people from the bus to share a cab with (it can get expensive doing so alone). Second was the Hue backpackers hostel located on Pham Ngu Lao ( This is the main backpacking district in Hue).
Hue has a lot of historical places. Unfortunately a lot of these have been destroyed during the war and are under restoration. The most important out of these is the Imperial City and the tombs of former emperors. The tombs are a little far away from the city so you probably need to rent a motorbike to see them.
All in all I would say, if you are short on time, this is one place you could skip.