  Enormous progress in physics enriched our knowledge about the particles which
build matter. Among them there are mesons, to which this thesis is entirely
devoted. The overwhelming majority of mesons is made of `conventional'
$q\bar{q}$ pairs, but nowadays there is mounting evidence for
`non-conventional' mesons, such as tetraquarks, glueballs, hybrids, and
molecules. The main aim of this thesis is to understand the nature of some
scalar and vector mesonic states which still remain puzzling. In this thesis we
investigate two nonets of $q\bar{q}$ excited vector mesons. Within an effective
QFT model we evaluate various decay channels of these states and compare the
results with experimental data from PDG. Moreover, we make predictions for a
not-yet observed $s\bar{s}$ state in $1$ $^3D_1$ nonet. Some non-conventional
mesons can be understood by the mechanism of dynamical generation. We show that
the inclusion of a single conventional $q\bar{q}$ seed in the relativistic QFT
Lagrangian may cause the appearance of an additional associated state as a
dynamically generated companion pole. Along this line, we show that light
scalar kaon $K^*_0(700)$ emerges as a companion pole of the $q\bar{q}$ meson
$K_0^*(1430)$. Moreover, we show that the enigmatic resonance $X(3872)$ in the
charmonium sector can be interpreted as the (virtual) companion pole of the
$c\bar{c}$ state $\chi_{c1}(2P)$. Then, we study the $c\bar{c}$ meson
$\psi(4040)$. We show that an additional companion pole appears on the complex
plane. At first sight one may identify it with the enhancement $Y(4008)$
observed by the Belle Collaboration. A detailed analysis reveals that different
mechanism is responsible for the creation of $Y(4008)$. Hence, the bump
associated with $Y(4008)$ should not be interpreted as an independent
resonance, but rather as a manifestation of the strong coupling of $\psi(4040)$
to the $D^*D$ channel.
