I see the word Senner/Sennen around a lot in the context of the Swiss. Who exactly are these people?

Senner (sing. Senn) or in French armaillis (sing. armailli, from Franco-provençal Gruyère patois armaye, meaning “cow”), are Alpine pastoral farmers, producing mostly milk and cheese (i.e. the famous Gruyère cheese) between poya (going up the Alps for summer grazing) and rindya (going back to the valley for the winter months).
Note that I have a tag dedicated to rindya (http://laurentbelkacem.tumblr.com/tagged/d%C3%A9salpe).
Even if they are no more than several hundred people in the whole country, they are still strong, traditional, popular figures in Vaud and Fribourg cantons, and that aura is very romantized by Swiss people (think Heidi with cows – an equivalent would be the Frontier Cowboy for the American soul).

Apart maybe in Jura (because they don’t have real mountains and are a bit jealous, and also are obsessed by making watches and fighting Bernese oppression) and Geneva (because outside there obsession for big black German SUVs and luxury watches, they tend to think the world revolves around their water-jet).
Culturally, the main Armaillis’ contribution to Swiss culture beside the light-blue edelweissed shirt is Father Bové’s Ranz des Vaches (https://youtu.be/zWWVMoV8UBY) which is often seen as an alternative/unofficial Swiss national hymn.

Galerie

BMW.

Vidéo

Bernard Romanens – Ranz des vaches (Lyôba/Call of the cows), an unofficial Swiss hymn (1977 video).

Lyôba, lyôba, por aryâ (bis).

Call of the cows for milking (bis) 

Vinyidè totè, byantsè, nêre, 

Come in all, white ones, black ones,

Rodzè, mothêlè, dzouvenè ôtrè, 

Red ones, starry ones, young ones and the others.

Dèjo chti tsâno, yô vo j’âryo, 

Under this oak where I milk you,

Dèjo chti trinbyo, yô i trintso,

Under this aspen where I make cheese

Lyôba, lyôba, por aryâ (bis).

Call of the cows, for milking (bis).

Vidéo

Bernard Romanens – Ranz des vaches (Lyôba/Call of the cows), an unofficial Swiss hymn (1977 video).

Lyôba, lyôba, por aryâ (bis).

Call of the cows for milking (bis) 

Vinyidè totè, byantsè, nêre, 

Come in all, white ones, black ones,

Rodzè, mothêlè, dzouvenè ôtrè, 

Red ones, starry ones, young ones and the others.

Dèjo chti tsâno, yô vo j’âryo, 

Under this oak where I milk you,

Dèjo chti trinbyo, yô i trintso,

Under this aspen where I make cheese

Lyôba, lyôba, por aryâ (bis).

Call of the cows, for milking (bis).