  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 Teichm\H{u}ller geodesic flow. In Part II we introduce the
$GL_2^+(\mathbb{R})$-action, and define the related notions of Veech group,
Teichm\H{u}ller 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.
