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Let X be a smooth projectibe curve over a finite field. We consider the Hall algebra H whose basis is formed by isomorphism classes of coherent sheaves on X and whose typical structure constant is the number of subsheaves in a given sheaf belonging to a given isomorphism class and with given isomorphism class of quotient. We study this algebra in detail. It turns out that its structure is strikingly similar to that of quantum affine algebras in their loop realization of Drinfeld. In this analogy the set of simple roots of a semisimple Lie algebra g corresponds to the set of cusp forms on X, the entries of the Cartan matrix of g to the values of Rankin-Selberg L-functions associated to two cusp forms. For the simplest instances of both theories (the curve P^1, the Lie algebra g=sl_2) the analogy is in fact an identity., plain TEX, 70 pages, no figures; misprints in some formulas are corrected and new references to relevant work added