  A relativistic quark model is applied to the description of semileptonic and
non-leptonic charmed decays of the B-meson. The exclusive semileptonic modes B
-> D l \nu and B -> D* l \nu are described through the universal Isgur-Wise
form factor, which is calculated in terms of a constituent quark model wave
function for the B-meson. Different approximations for the latter, either based
on a phenomenological ansatz or derived from analyses of the meson spectra, are
adopted. In particular, two wave functions, constructed via the Hamiltonian
light-front formalism using a relativized and a non-relativistic constituent
quark model, are considered, obtaining a link between standard spectroscopic
quark models and the B-meson decay physics. The inclusive semileptonic and
non-leptonic branching ratios are calculated within a convolution approach,
inspired by the partonic model and involving the same B-meson wave function
used for the evaluation of the exclusive semileptonic modes. Our results for
the major branching ratios are consistent with available experimental data and
the sum of all the calculated branching ratios turns out to be close to unity.
In particular, we found that: i) a remarkable fraction (~ 35%) of semileptonic
decay modes occur in non-D, non-D* final states; ii) non-perturbative effects
enhance the inclusive b -> c \bar{u} d decay channels, with a sizable
contribution provided by internal decays into heavy mesons and
baryon-antibaryon pairs; the resulting reduction of the semileptonic branching
ratio brings the theoretical prediction in agreement with the experimental
value without increasing at the same time the charm counting.
