If we have moved onto the Ethnicity of the Lao people, my Insight Guide gives lots of interesting information:-
The (approximately) half the total population who are ethnic Lao, known locally as Lao Lum, are closely related to the Lao speaking inhabitants of north-east Thailand, and slightly more distantly to the Thai people themselves. These are the people of the Mekong Vally lowlands who predominate in the provinces of Lunag Prabang, Ventiane, Tha Kaek, Savannakhet and Pakse.
The distinction between Lao and Thai is rather indistinct and something of a new (and politically motivated) phenomenon. Certainly the two groups are part of the same family, something both sides will happily accept - yet the Lao can be irritated by the rather arrogant and frequently stated Thai contention that the Lao are their "little brothers".
Of the remaining half of the population, an estimated 20% comprise Lao Tai groups, such as Tai Dam, Tai Daeng and Tai Khao, all ethnic Tai sub groups. All these groups are closely related to the Lao Lum, but live higher up in the hills and cultivate dry rice, as opposed to the irrigated rice paddy culture of the lowland Mekong valley.
Next there are the Lao Theung, or "approaching the top of the mountain Lao", a loose affiliation of mostly Mon-Khmer people who live halfway up the mountains. Formerly known to the ruling Lao Lum by the perjorative term 'kha', or slave, this group constitutes a further 15-20% of the population (and includes my GF - who originated from outside Luang Prabang and who tells me that the language also varies between these different groups). This group makes up by far the most economically disadvantaged section of Lao society.
Finally, on the distant mountain tops live, as might be expected - the Lao Sung, or "High Lao", people whose communities are at altitudes of more than 1000 metres above sea level.
In official terms the Lao Lum, and Lao Tai constitute 60% of the population, the Lao Theung a further 34% of the population, and the Lao Sung, the remainder. These three major groups, Lao Sung, Lum and Theung are depicted, from left to right, on the back of the Lao PDR 1000 kip bank note. (My GF is the short one on the right - Lao Theung!

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Given the considerable numbers of Vietnamese and Chinese in the country, the figures above must be considered applicable only to the "indigenous Lao people".
My GF, told me that when living in her village in Luang Prabang her family spoke Lao Theung most of the time, but after moving to Ventianes they took to speaking Lao Lum, as it was the predominant language there and because they found they were treated better/more favourably.