Ideal triangles, hyperbolic surfaces and the Thurston metric on Teichm{ü}ller space

These are notes on the hyperbolic geometry of surfaces, Teichm{ü}ller spaces and Thurston's metric on these spaces. They are associated with lectures I gave at the Morningside Center of Mathematics of the Chinese Academy of Sciences in March 2019 and at the Chebyshev Laboratory of the Saint Petersburg State University in May 2019. In particular, I survey several results on the behavior of stretch lines, a distinguished class of geodesics for Thurston's metric and I point out several analogies between this metric and Teichm{ü}ller's metric. Several open questions are addressed. The final version of these notes will appear in the book "Moduli Spaces and Locally Symmetric Spaces", edited by L. Ji and S.-T. Yau, International Press, 2021.