It is a fundamental problem in algebraic geometry to study equations defining algebraic curves. In 1984, Mark Green formulated a famous conjecture on equations defining canonical curves and their syzygies. In early 2000's, Claire Voisin made a major breakthrough by proving that Green's conjecture holds for general curves. A few years ago, Aprodu-Farkas-Papadima-Raicu-Weyman gave a new proof of Voisin's theorem by studying equations defining tangent developable surfaces, and recently, I obtained a simple geometric proof of their result using equations defining secant varieties. In this talk, I first review the geometry of algebraic curves, and then, I explain the main ideas underlying the recent work on syzygies of algebraic curves and their tangent and secant varieties.