See Toen course in cours folder under Toen, chapter 7.
[arXiv:0912.4551] Torsors, herds and flocks from arXiv Front: math.CT by Thomas Booker, Ross Street 1 person liked this This paper presents non-commutative and structural notions of torsor. The two are related by the machinery of Tannaka-Krein duality.
nLab page on Torsor