Motivated by issues in string theory and M-theory, we provide a pedestrian
introduction to automorphic forms and theta series, emphasizing examples rather
than generality.Comment: 22 pages, to appear in the Proceedings of Les Houches Winter School
``Frontiers in Number Theory, Physics and Geometry'', March 9-21, 2003; v2:
minor changes and clarifications, section 3.5 on pure spinors has been
rewritte