Elementary introduction to pre-big bang cosmology and to the relic graviton background

This is a contracted version of a series of lectures for graduate and undergraduate students given at the {\sl "VI Seminario Nazionale di Fisica Teorica"} (Parma, September 1997), at the Second Int. Conference {\sl "Around VIRGO"} (Pisa, September 1998), and at the Second {\sl SIGRAV} School on {\sl "Gravitational Waves in Astrophysics, Cosmology and String Theory"} (Center "A. Volta", Como, April 1999). The aim is to provide an elementary, self-contained introduction to string cosmology and, in particular, to the background of relic cosmic gravitons predicted in the context of the so-called "pre-big bang" scenario. No special preparation is required besides a basic knowledge of general relativity and of standard (inflationary) cosmology. All the essential computations are reported in full details either in the main text or in the Appendices. For a deeper and more complete approach to the pre-big bang scenario the interested reader is referred to the updated collection of papers available at {\tt http://www.to.infn.it/\~{}gasperin/}.