Champasak is a province of Southern Laos. It's best known for a series of Khmer ruins, collectively named a UNESCO World Heritage Site as the Champasak Cultural Landscape.
In ancient times, the area belonged to the realm of Chenla, one of its capitals Shrestapura was presumably located at Vat Phou. From the 10th to 13th century, it was part of the Khmer Empire. Later, the region was populated by Kuy people who speak a language distantly related to Khmer.
The Lao proper only arrived around 1700 when, after the disintegration of the Lan Xang Empire, people from Vientiane migrated south and settled in Champasak. The new polity was first ruled by a popular monk and later by a daughter of Lao king Sulinyavongsa. In 1713, Champasak was recognised as the third Lao kingdom after Luang Prabang and Vientiane. It was subdued by its powerful neighbour Siam in 1778.
After the Franco-Siamese War of 1893, Champasak was separated: the left bank of the Mekong became a French protectorate, the western parts (including the capital) remained under Siamese overlordship, until they too were annexed to French Indochina in 1904. Pakse was founded by the French in 1905 as the new administrative centre. Prince Boun Oum of Champasak relinquished his right to rule in 1945, in favour of a unified Kingdom of Laos. In the Lao civil war, starting 1958, he was the head of the rightist faction that had its stronghold in Champasak.
With 45 inhabitants per square kilometer, Champasak is one of the most densely populated areas of Laos (which does not mean a lot, as the country as a whole is very sparsely peopled). The development index is slightly above Lao average, but considerably higher than in the neighbouring provinces of Salavan and Attapeu.
Pakse is the transport hub of the region, with an airport (IATA: PKZ), bus service and limited boat access off the Mekong. For Champasak and the south, there are more expensive minivans (110,000 kip return to Si Pan Don) from a couple of travel companies ('Pakse Travel' and 'Explore Asia') in Pakse town centre (nearly always packed with bleach blond 20-somethings. Don't expect any leg room).
From Pakse southern bus station, a songthaew 20,000 kip) to Champasak (20,000 kip, ~2 hr) or Than Bhan Khop (2 hr 15 min), to near the crossing to Wat Phu (30-min walk to Wat Phu entrance).
Most places in Champasak, including Wat Phou, can be visited as day trips from Pakse.
There are restaurants at many hotels. Street vendors can be found along the road to Wat Phu.
It seems that buses from Champasak-Pakse are only available in the morning, 07:00-08:00, 20,000 kip.