Let us consider a compact oriented riemannian manifold M without boundary and
of dimension n=4k. The signature of M is defined as the signature of a given
quadratic form Q. Two different products could be used to define Q and they
render equivalent definitions: the exterior product of 2k-forms and the cup
product of cohomology classes. The signature of a manifold is proved to yield a
topological invariant. Additionally, using the metric, a suitable Dirac
operator can be defined whose index coincides with the signature of the
manifold. This second version includes corrections and many examples.