  The mass spectra of heavy and light mesons is computed within the framework
of the relativistic flux tube model. A good agreement with the experimental
data is obtained provided that the flux tube contributions, including
retardation and spin-orbit effects, are supplemented by a one-gluon-exchange
potential, a quark self-energy term and instanton-induced interactions. No
arbitrary constant is needed to fit the absolute scale of the mass spectra, and
the different parameters are fitted on lattice QCD in order to strongly
restrict the arbitrariness of our model. The relevance of the present approach
is discussed in the case of glueballs, and the glueball spectrum we compute is
compared to the lattice QCD one. Finally, we make connections between the
results of our model and the nature of some newly discovered experimental
states such as the f_0(1810), X(3940), Y(3940), etc.
