In this talk, we will review the definition of 2-dimensional almost-Riemannian structures (2-ARSs) from several different viewpoints. Generic 2-ARSs have only three types of points: ordinary points, Grushin points, and tangency points. We will introduce some results on the curves admitting generic singular points. Then we will construct the duality between 2-ARSs and coherent tangent bundles of rank 2. Furthermore, we will give the correspondence of singular points, metrics, and curvatures between the two structures.