  In this lecture we briefly review the definition, consequences and
applications of an entropy, $S_q$, which generalizes the usual Boltzmann-Gibbs
entropy $S_{BG}$ ($S_1=S_{BG}$), basis of the usual statistical mechanics, well
known to be applicable whenever ergodicity is satisfied at the microscopic
dynamical level. Such entropy $S_q$ is based on the notion of $q$-exponential
and presents properties not shared by other available alternative
generalizations of $S_{BG}$. The thermodynamics proposed in this way is
generically {\it nonextensive} in a sense that will be qualified. The present
framework seems to describe quite well a vast class of natural and artificial
systems which are not ergodic nor close to it. The a priori calculation of $q$
is necessary to complete the theory and we present some models where this has
already been achieved.
