Résumé
We review the different notions about translation surfaces which are necessary to understand McMullen's classification of$GL_2^+(\mathbb{R})$ -orbit closures in genus two. In Section 2 we recall the different definitions of a translation surface, in increasing order of abstraction, starting with cutting and pasting plane polygons, ending with Abelian differentials. In Section 3 we define the moduli space of translation surfaces and explain its stratification by the type of zeroes of the Abelian differential, the local coordinates given by the relative periods, its relationship with the moduli space of complex structures and the Teichmuller geodesic flow. In Part II we introduce the$GL_2^+(\mathbb{R})$ -action, and define the related notions of Veech group, Teichmuller disk, and Veech surface. In Section 9 we explain how McMullen classifies$GL_2^+(\mathbb{R})$ -orbit closures in genus$2$ : you have orbit closures of dimension$1$(Veech surfaces, of which a complete list is given),$2$(Hilbert modular surfaces, of which again a complete list is given), and$3$(the whole moduli space of complex structures). In the last section we review some recent progress in higher genus.