From the World Heritage inscription
Situated on the edge of a plateau that dominates the plain of Cambodia, the Temple of Preah Vihear is dedicated to Shiva. The Temple is composed of a series of sanctuaries linked by a system of pavements and staircases over an 800 metre long axis and dates back to the first half of the 11th century AD. Nevertheless, its complex history can be traced to the 9th century, when the hermitage was founded. This site is particularly well preserved, mainly due to its remote location. The site is exceptional for the quality of its architecture, which is adapted to the natural environment and the religious function of the temple, as well as for the exceptional quality of its carved stone ornamentation.
I can’t recommend visiting Preah Vihear until the tensions between Cambodia and Thailand cool down. Even then, the best route is to visit via Thailand, not from Cambodia, even though it technically lies in Cambodia. There are no paved roads on the Cambodian side of the border, but on the Thai side there is very easy access and tour buses which frequent the temple.
I hope that one day there is enough development in Cambodia that visitors to Angkor in Sieam Reap will be able to easily visit Preah Vihear. Currently, it isn’t worth the effort.