  The existence and properties of the sigma meson have been controversial for
almost six decades, despite playing a central role in the spontaneous chiral
symmetry of QCD or in the nucleon-nucleon attraction. This controversy has also
been fed by the strong indications that it is not an ordinary quark-antiquark
meson. Here we review both the recent and old experimental data and the model
independent dispersive formalisms which have provided precise determinations of
its mass and width, finally settling the controversy and leading to its new
name: $f_0(500)$. We then provide a rather conservative average of the most
recent and advanced dispersive determinations of its pole position
$\sqrt{s_\sigma}=449^{+22}_{-16}-i(275\pm12)$.
  In addition, after comprehensive introductions, we will review within the
modern perspective of effective theories and dispersion theory, its relation to
chiral symmetry, unitarization techniques, its quark mass dependence, popular
models, as well as the recent strong evidence, obtained from the QCD $1/N_c$
expansion or Regge theory, for its non ordinary nature in terms of quarks and
gluons.
