Ngari prefecture (also known as Ali prefecture 阿里地区 Ālǐ Dìqū) is in the far west of Tibet, it's an extremely remote area with nearly no infrastructure of any type. Nicknamed the "Top of the Roof of the World".
Ngari Gunsa Airport is 40 km away from the town of Ali. Destinations include Lhasa and Kashgar.
Ngari's capital Ali is on G219 Xinjiang–Tibet Highway. Road condition is good from Lhasa to Ali. As of 2012, road beyond Rutog is still under construction, with unpaved sections here and there. The road will be completely paved in 2013.
Domestic travelers: Ngari is restricted area. A Border Permit (边境通行证) is required. The permit can be arranged in your home city, or you can also get the permit in Lhasa or Kashgar.
Foreign travelers: As of 2012, foreign tourists must travel to Tibet in a group of 3+ with the same nationality.
The main safety concerns relate to the extreme remoteness of the area. Always bring a couple days of food with you when traveling outside of a village, and carry a few liters of water when traveling by truck, (The driver will probably have a big jerry can, but don't be dependent on it). In summer it can be warm when the sun is out, but change as soon as it's overcast to wintery conditions. Hail is common, and snow not unknown in the middle of summer. Bring a good sleeping bag, and warm cloths.